Electrocatalytic oxidation of methanol on Cu modified polyaniline electrode in alkaline medium

Electrolytically deposited Cu on polyaniline film covered Pt substrate (Cu/PANI/Pt) is used as anode for the electrooxidation of methanol in alkaline medium. The electrochemical behavior and electrocatalytic activity of the electrode were characterized using cyclic voltammetry, impedance spectroscopy, chronomethods, rotating disc voltammetry and polarization studies. The morphology and composition of the modified film were obtained using SEM and EDAX techniques. The electrooxidation of methanol in NaOH is found to be more efficient on Cu/PANI/Pt than on bare Cu (Cu), electrodeposited Cu on Cu (Cu/Cu) and electrodeposited Cu on Pt (Cu/Pt) substrates. Partial chemical displacement of dispersed Cu on PANI with Pt or Pd further improved its performance towards methanol oxidation.     

Electro-oxidation of methanol on copper in alkaline solution

The electro-oxidation of methanol on copper in alkaline solutions has been studied by the methods of cyclic voltammetry, quasi-steady state polarization and chronoamperometry. It has been found that in the course of an anodic potential sweep the electro-oxidation of methanol follows the formation of Cu^III and is catalysed by this species through a mediated electron transfer mechanism. The reaction also continues in the early stages of the reversed cycle until it is stopped by the prohibitively negative potentials. The process is diffusion controlled and the current-time responses follow Cottrellian behavior. The rate constants, turnover frequency, anodic transfer coefficient and the apparent activation energy of the electro-oxidation reaction are reported.     

Complete oxidation of methane on Pd/YSZ and Pd/CeO2/YSZ by electrochemical promotion

In this work, methane combustion over Pd/YSZ and Pd/CeO₂/YSZ catalyst was investigated at a temperature range of 470–600 °C. For the first time, the feasibility of electrochemical promotion on palladium films prepared by wet impregnation was reported. The catalytic activity of palladium was found to increase over 160% via transference of oxygen ions from the solid electrolyte to the catalyst film. In addition, palladium supported over ceria and yttria-stabilized zirconia showed the highest activity. As expected, the presence of ceria allowed improving the oxygen storage capacity of the catalyst system.     

Enhanced electrocatalytic oxidation of methanol on Pd/polypyrrole–graphene in alkaline medium

The new electrocatalyst of Pd nanoparticles supported on polypyrrole-functionalized graphene (Pd/PPy–graphene) was reported. Microstructure, morphology and crystallinity of the synthesized materials were investigated by Fourier transform infrared spectrometry (FT-IR), scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The elements of the composite were characterized using energy dispersive X-ray spectroscopy (EDS). The results showed that the polypyrrole-wrapped graphene support successfully discrete Pd nanoparticles with the crystallite size of about 6 nm. Catalyst activity for methanol electro-oxidation in fuel cells was investigated by cyclic voltammetry (CV) and chronoamperometry. The fundamental electrochemical test results indicated that the electrocatalytic activity of Pd/PPy–graphene is much better than that of commercial catalyst for methanol oxidation.     

Electrochemical effect of gold nanoparticles on Pt/α-Fe2O3/C for use in methanol oxidation in alkaline solution

A catalyst containing gold nanoparticles with Pt/α-Fe₂O₃/C was prepared by a co-precipitation method and its catalytic activity for the oxidation of methanol, formaldehyde, and formic acid in alkaline solutions was evaluated by an electrochemical method and high-performance liquid chromatography (HPLC). The addition of gold nanoparticles improved catalytic activity only for the oxidation of methanol and formaldehyde, and not for the oxidation of formic acid. HPLC analysis was performed for methanol oxidation to detect the oxidative products. In HPLC analysis, only formate anion could be detected in the electrolyte solution and the ratio of formate anion obtained to the total passed charge in Pt/nano-Au/α-Fe₂O₃/C was less than that in Pt/C, indicating that formic acid is not the final product of methanol oxidation. These results show that gold nanoparticles promoted methanol oxidation up to CO₂.     

Poly(o-toluidine) as the matrix for incorporation of palladium species from PdCl2 aqueous solutions

Reactivity of poly(o-toluidine) in the emeraldine base form (POT) and protonated with HCl (POT/HCl) in PdCl₂ aqueous solutions of various HCl concentrations has been studied. Using elemental analysis, FTIR, UV–Vis, XPS and EXAFS spectroscopies as well as XRD and SEM it has been established that POT/HCl is more reactive than POT. The course of reactions is influenced by the type of the PdCl₂ solution. Thus, protonation of POT with incorporation of palladium (II) chloro–and/or aquachloro–and/or chlorohydroxycomplexes counterions is the main process occurring in the PdCl₂ solutions of higher HCl concentration. A redox reaction resulting in the oxidation of the polymer chain with simultaneous formation of metallic palladium takes place in the PdCl₂ solution of lower HCl concentration. POT/HCl shows enhanced reducing properties with respect to POT. Lowering of the protonation level (i.e. some deprotonation) of POT/HCl has been also observed. Coordination of palladium (II) ions by nitrogen atoms of the polymer chain can be also postulated.     

Influence of BaO on Pd/Al2O3-based catalysts in C2H4 and CO oxidation as well as in NO reduction

In this study, Pd/Al₂O₃ and Pd/BaO/Al₂O₃ metallic monoliths were used to investigate the effect of BaO in C₂H₄ and CO oxidation as well as in NO reduction. A FT-IR gas analyser was used to study the activity of the catalysts. Several activity experiments carried out with dissimilar feedstreams revealed that BaO enhances CO and C₂H₄ oxidation as well as NO reduction reactions in rich conditions. This effect is due to BaO, which causes a decrease in the ethene poisoning of palladium. In lean conditions BaO is present in the form of Ba(OH)₂ which reacts with oxidised NO releasing water. Therefore, NO was stored during the lean reaction.     

Electrochemical impedance spectroscopic investigation of CO2 reduction on polyaniline in methanol

The ac response of polyaniline thin films on platinum electrodes was measured at different dc potentials during the CO₂ reduction in methanol/LiClO₄ electrolyte with a small amount of 0.5 M H₂SO₄. The complex capacitance curves were simulated and the data obtained were used to calculate kinetic parameters, based on the assumption that the thermodynamic potential E₀ is in the region of −0.2-−0.1 V versus saturated calomel electrode (SCE). With E₀=−0.2 V versus SCE and β=0.6, a j₀ value of ca. 10⁻⁴ A cm⁻² was found for the electroreduction of CO₂ on the polyaniline electrode.     

Transformation of methane formation sites into methanol formation ones during CO---H2 reaction over Pd/CeO2 in its SMSI state

The phenomena of induction period of methanol formation, observed in CO---H₂ reaction over Pd/CeO₂ catalysts under SMSI state, was investigated in depth. The magnitude of the induction period was dependent on the extent of SMSI, and higher temperature H₂ reduction lengthened it accompanied with the increase of the number of active sites for methane formation. On the contrary, by the pretreatment of SMSI surface with water vapor, this induction period almost disappeared with the drastic decrease of methane formation rate. These results indicate that methane formation sites would be transformed into methanol formation sites by the oxidation of water vapor formed during CO---H₂ reaction. Infrared spectroscopic investigation of adsorbed CO after various pretreatments indicated that during the induction period thin layers of reduced ceria, which preferentially covered Pd(1 1 1) plane under SMSI state, were removed from the Pd(1 1 1) plane by formed water vapor during CO---H₂ reaction. It was concluded that Pd(1 1 1) plane adjacent to ceria would be the efficient active sites for methanol formation.     

Effects of incorporation of Cu and Ag in Pd on electrochemical oxidation of methanol in alkaline solution

Pd fine particles were prepared by heterogeneous reaction of PdOx with dry methanol as well as by the NaBH4 reduction method. The former method was found to give Pd nanoparticles (~5nm). Similarly f.c.c. structured, single phase nanoparticles of alloy compositions Pd0.8Cu0.2, Pd0.5Cu0.5, Pd0.8Ag0.2 and Pd0.5Ag0.5 were prepared by the heterogeneous reaction of dry methanol with intimate mixtures of PdOx+CuOx and PdOx+AgNO3. The electrochemical properties of the porous unsupported electrodes, prepared from these materials, in alkaline solutions, were investigated by cyclic voltammetry and steady-state polarization measurements. Various processes taking place during potential scanning in the presence and absence of methanol in 6m KOH solution arediscussed. Steady-state polarization data indicate that the methanol oxidation reaction (MOR) activity decreases with incorporation of Cu and Ag into the Pd lattice. The extent of decrease in the MOR activity is less for Cu ad dition than for Ag addition.     

Hydrogen production by oxidative methanol reforming on Pd/ZnO catalyst: effects of Pd loading

Catalytic performances of Pd/ZnO in oxidative methanol reforming reaction were studied as a function of Pd loading. It was confirmed that the formation of Pd–Zn alloy is essential to the selective production of hydrogen. High active Pd/ZnO, comparable to commercial Cu-Zn catalyst, was obtained with higher Pd loading. Selectivity of the reaction was greatly increased by increasing Pd loading on ZnO. At higher Pd loadings (>5%), co-precipitation was superior to impregnation for the catalyst preparation. The catalytic performances were also discussed based on results from X-ray diffraction (XRD) characterization.     

Influence of bismuth on the structure and activity of Pt and Pd nanocatalysts for the direct electrooxidation of NaBH4

In the past few years, borohydrides have gathered a lot of attention as an energy carrier for fuel cell application. Numerous investigations on both hydrogen generation and direct oxidation of NaBH₄ have been published. Nonetheless, in our knowledge, only a few catalysts are capable to completely perform the direct oxidation of NaBH₄ at low potentials without hydrogen evolution.In this work, carbon supported Pd_1−xBi_x/C and Pt_1−xBi_x/C nanocatalysts were synthesized by a “water in oil” microemulsion method. The influence of surface modifications of Pt and Pd by Bi on the electrooxidation of sodium borohydride in alkaline media was evaluated. Physical and electrochemical methods were applied to characterize the structure and surface of the synthesized catalysts.It was verified that bismuth is present at the surface of the bimetallic catalysts and that hydrogen adsorption/desorption reactions are strongly limited on Pt and Pd surfaces with high bismuth coverage. Although the onset potential for NaBH₄ oxidation on Pd_xBi_1−x/C catalysts is ca. 0.2 V higher than that for Pd/C, the presence of bismuth on palladium surface influences the reaction mechanism, limiting hydrogen evolution and oxidation in the case of Pd_0.8Bi_0.2 catalyst. On Pt_0.9Bi_0.1 catalyst the onset potential remains unchanged comparing to Pt/C and negligible hydrogen evolution was observed in the whole potential range where the catalyst is active. The number of exchanged electrons was calculated using the Koutecky-Levich equation and it was found that for Pt_0.9Bi_0.1 catalyst, ca. 8 electrons are exchanged per BH₄⁻ ion at low potentials. The presented results are remarkable evidencing that NaBH₄ can be directly oxidized at low potentials with high energy efficiency.     

Oxide (CeO2, NiO, Co3O4 and Mn3O4)-promoted Pd/C electrocatalysts for alcohol electrooxidation in alkaline media

This study investigated Pt/C, Pd/C and oxide (CeO₂, NiO, Co₃O₄ and Mn₃O₄)-promoted Pd/C for electrooxidation reactions of methanol, ethanol, ethylene glycol and glycerol in alkaline media. The results show that Pd/C electrocatalysts alone have low activity and very poor stability for the alcohol electrooxidation. However, addition of oxides like CeO₂, NiO, Co₃O₄ and Mn₃O₄ significantly promotes catalytic activity and stability of the Pd/C electrocatalysts for the alcohol electrooxidation. The Pd-Co₃O₄ (2:1, w:w)/C shows the highest activity for the electrooxidation of methanol, EG and glycerol while the most active catalyst for the ethanol electrooxidation is Pd-NiO (6:1, w:w)/C. On the other hand, Pd-Mn₃O₄/C shows significantly better performance stability than other oxide-promoted Pd/C for the alcohol electrooxidation. The poor stability of the Pd-Co₃O₄/C electrocatalysts is most likely related to the limited solubility of cobalt oxides in alkaline solutions.     

CoPdx oxygen reduction electrocatalysts for polymer electrolyte membrane and direct methanol fuel cells

The electrochemical activity of carbon-supported cobalt-palladium alloy electrocatalysts of various compositions have been investigated for the oxygen reduction reaction in a 5 cm² single cell polymer electrolyte membrane fuel cell. The polarization experiments have been conducted at various temperatures between 30 and 60 °C and the reduction performance compared with data from a commercial Pt catalyst under identical conditions. Investigation of the catalytic activity of the CoPd_x PEMFC system with varying composition reveals that a nominal cobalt-palladium atomic ratio of 1:3, CoPd₃, exhibits the best performance of all studied catalysts, exhibiting a catalytic activity comparable to the commercial Pt catalyst. The ORR on CoPd₃ has a low activation energy, 52 kJ/mol, and a Tafel slope of approximately 60 mV/decade, indicating that the rate-determining step is a chemical step following the first electron transfer step and may involve the breaking of the oxygen bond. The CoPd₃ catalyst also exhibits excellent chemical stability, with the open circuit cell voltage decreasing by only 3% and the observed current decreasing by only 10% at 0.8 V over 25 h. The CoPd₃ catalyst also exhibits superior tolerance to methanol crossover poisoning than Pt.     

Electro-oxidation of methanol and ethylene glycol on platinum in alkaline solution: Poisoning effects and product analysis

The electrochemical oxidation of ethylene glycol on platinum was investigated and compared with that of methanol in alkaline solution by using various electrochemical and analytical measurements. Ethylene glycol showed much less significant electrode poisoning than methanol at low potential (400 mV). This phenomenon was clarified by analyzing the products of ethylene glycol oxidation. In ethylene glycol oxidation, partial oxidation to glycolate was much faster than complete oxidation to CO₂. In addition, there were two paths for ethylene glycol oxidation: poisoning and non-poisoning paths. The poisoning path led to the production of C1 compounds and the non-poisoning path gave oxalate. The non-poisoning path prevented the formation of poisonous species on platinum.     

Oxidation state of palladium as a factor controlling catalytic activity of Pd/SiO2–Al2O3 in propane combustion

The oxidation state of palladium on SiO₂–Al₂O₃ used for propane combustion was examined by XPS and XRD, and the correlation of the catalytic activity with the oxidation state of palladium was systematically studied. The propane conversion over 5 wt% Pd/SiO₂–Al₂O₃ was measured in the range 1.0≤S≤7.2 (S is defined as [O₂]/5[C₃H₈] based on stoichiometric ratio). The propane conversion strongly depended on the S value and reached the maximum at S=5.5. The oxidation state of palladium also changed with the S value; palladium particles were more oxidized under the reaction mixture of higher S value. On the sample used for the reaction at S=5.5, both of metallic palladium and palladium oxide were found. It is concluded that partially oxidized palladium which has optimum ratio of metallic palladium to palladium oxide shows the highest catalytic activity in propane combustion.     

Influence of Pd on the structure and electrochemical hydrogen storage properties of Mg50Ti50 alloy prepared by ball milling

High-energy ball milling was used to modify the physico-chemical and the electrochemical hydrogenation properties of Mg₅₀Ti₅₀ alloy via the addition of Pd. This was done by first ball milling Mg and Ti together for (20 − x) hours. 3.3 at.% Pd was then added and ball milling was resumed for x hours. X-ray diffraction and X-ray photoelectron spectroscopy analyses revealed that the alloying of Pd with pre-milled Mg₅₀Ti₅₀ was initiated after only a few minutes and was completed after 5 h of milling. The maximum discharge capacity of the Mg₅₀Ti₅₀-3.3 at.% Pd electrode increased significantly with the milling time (from 35 mAh g⁻¹ for 5 min to 480 mAh g⁻¹ for 20 h of milling). The exchange current density increased with the milling time and was directly related to the Pd surface concentration, suggesting that Pd plays a key role in facilitating the charge-transfer reaction. In contrast, the incorporation of Pd had a minor effect on the hydrogen diffusion coefficient. The electrochemical pressure-composition isotherms revealed a significant destabilization of the hydride as the milling time with Pd increased. No significant improvement in the hydrogen storage properties of Mg₅₀Ti₅₀-Pd electrodes was observed for Pd concentrations higher than 3.3 at.%.     

H2 sorption in HCl-treated polyaniline and polypyrrole

Hydrogen sorption in conducting polymers was investigated in order to determine their potential as hydrogen storage media. The conducting polymers, polyaniline and polypyrrole, were treated with an acid, which resulted in an exceptionally high hydrogen sorption, 6 and 8 wt% at room temperature and under 9.3 MPa. Both the molecular effect and electrical effects by the conducting polymers appear to play an important role in hydrogen sorption. This paper presents the preliminary results of hydrogen sorption in a conducting polymer along with its characterization by XRD, scanning electron microscopy, TGA, and conductivity measurement using a four-probe method. A possible mechanism for the extraordinarily high hydrogen storage is suggested.     

Oxygen reduction at Pt and Pt70Ni30 in H2SO4/CH3OH solution

The electrochemical oxygen reduction reaction (ORR) was studied at Pt and Pt alloyed with 30 atom% Ni in 1 M H₂SO₄ and in 1 M H₂SO₄/0.5 M CH₃OH by means of rotating disc electrode. In pure sulphuric acid, the overpotential of ORR at 1 mA cm⁻² is about 80 mV lower at Pt₇₀Ni₃₀ than at pure Pt. It was found that in methanol containing electrolyte solution the onset potential for oxygen reduction at PtNi is shifted to more positive potentials and the alloy catalyst has an 11 times higher limiting current density for oxygen reduction than Pt. Thus, PtNi as cathode catalyst should have a higher methanol tolerance for fuel cell applications. On the other hand, no significant differences in the methanol oxidation on both electrodes was found using cycling voltammetry, especially regarding the onset potential for methanol oxidation. During all the measurements no significant electrochemical activity loss was observed at Pt_0.7Ni_0.3. Ex-situ XPS investigations before and after the electrochemical experiments have revealed Pt enrichment in the first surface layers of the PtNi.     

Influence of the nanostructure on charge transport in polyaniline films

Two kinds of polyaniline films with different microstructures were prepared and investigated. One of the films was prepared with polyaniline having nanofiber structure, which was synthesized by rapidly mixed reaction; and the other was prepared with polyaniline having granular particle structure, synthesized by conventional chemical oxidation method. The spectroscopic, electrochemical, and transport properties of the two kinds of films were studied with an interest in an influence of film microstructure on mobilities of charge carriers in polymer films. A clear difference was observed in the mobility versus oxidation level plot between the two kinds of polyaniline films. The results show that nanofiber structure of polyaniline film enhances mobilities, but affect little features of the mobility change induced by oxidation, which implies that although the nanofibers structure facilitates the charge transport in neutral or oxidized films, but it is irrelevant to the evolution of metallic conduction in polyaniline films.