Electrodeposition of mesoscopic Pt-Ru on reticulated vitreous carbon from reverse emulsions and microemulsions: Application to methanol electro-oxidation

High surface area Pt-Ru (between 120 and 400 cm² mg⁻¹) meso-sized particles and mesoporous coatings were electrodeposited on reticulated vitreous carbon (RVC) three-dimensional electrodes using reverse emulsions and microemulsions. The organic phase of the colloidal media was composed of cyclohexane, Triton X-100 non-ionic surfactant and tetrabutylammonium perchlorate (for ionic conductivity) while the aqueous phase contained H₂PtCl₆ and RuCl₃ (or (NH₄)₂RuCl₆). For microemulsification to occur isopropanol was also added as co-surfactant. The catalytic activity for the electro-oxidation of methanol was assessed by cyclic voltammetry and chronopotentiometry in conjunction with surface area measurement by Cu underpotential deposition. The composition and morphology of the Pt-Ru deposit was analyzed by inductively coupled plasma atomic emission spectroscopy and scanning electron microscopy, respectively. The effects on the catalytic activity of the deposition current density, temperature, RVC pretreatment and plating bath composition are presented. It was found that the electrodeposition of Pt-Ru in reverse microemulsion yielded the highest specific surface area (400 cm² mg⁻¹) and catalytic activity toward CH₃OH electro-oxidation as shown, for example, by a 50-200 mV more negative anode potential determined by chronopotentiometry compared to a catalyst obtained by pure aqueous and emulsion electroplating.     

Electrochemically assisted organosol method for Pt-Sn nanoparticle synthesis and in situ deposition on graphite felt support: Extended reaction zone anodes for direct ethanol fuel cells

Two electrochemically assisted variants of the Bönneman organosol method were developed for Pt-Sn nanoparticle synthesis and in situ deposition on graphite felt electrodes (e.g. thickness up to 2 mm). Tetraoctylammonium triethylhydroborate N(C₈H₁₇)₄BH(C₂H₅)₃ was employed as colloid stabilizer and reductant dissolved in tetrahydrofuran (THF). The role of the electric field at a low deposition current density of 1.25 mA cm⁻² was mainly electrophoretic causing the migration and adsorption of N(C₈H₁₇)₄BH(C₂H₅)₃ on the graphite felt surface where it reduced the PtCl₂-SnCl₂ mixture. Faradaic electrodeposition was detected mostly for Sn. Typical Pt-Sn loadings were between 0.4 and 0.9 mg cm⁻² depending on the type of pre-deposition exposure of the graphite felt: surfactant-adsorption and metal-adsorption variant, respectively. The catalyst surface area and Pt:Sn surface area ratio was determined by anodic striping of an underpotential deposited Cu monolayer. The two deposition variants gave different catalyst surfaces: total area 233 and 76 cm² mg⁻¹, with Pt:Sn surface area ratio of 3.5:1 and 7.7:1 for surfactant and metal adsorption, respectively. Regarding electrocatalysis of ethanol oxidation, voltammetry and chronopotentiometry studies corroborated by direct ethanol fuel cell experiments using 0.5 M H₂SO₄ as electrolyte, showed that due to a combination of higher catalyst load and Pt:Sn surface ratio, the graphite felt anodes prepared by the metal-adsorption variant gave better performance. The catalyzed graphite felt provided an extended reaction zone for ethanol electrooxidation and it gave higher catalyst mass specific peak power outputs compared to literature data obtained using gas diffusion anodes with carbon black supported Pt-Sn nanoparticles.     

Nickel-dimethylglyoxime complex modified graphite and carbon paste electrodes: preparation and catalytic activity towards methanol/ethanol oxidation

Nickel-dimethylglyoxime complex (abbreviated as Ni(II)(DMG)₂) modified carbon paste and graphite electrodes were prepared by mixing Ni(II)(DMG)₂ with graphite paste, and coating Ni(II)(DMG)₂ to the graphite surface. It is necessary to cycle the electrode potential to a high value (e.g. 0.8 V versus SCE) for the preparation of the modified electrodes. The electrochemical reaction was originally assumed to be a one-electron process converting Ni(II)(DMG)₂ to [(DMG)₂(H₂O)Ni(III)ONi(III)(OH)(DMG)₂]⁻. [(DMG)₂(H₂O)Ni(III)ONi(III)(OH)(DMG)₂]⁻ showed a strong catalytic activity toward electro-oxidation of methanol and ethanol. The electrocatalytic oxidation currents consistently increase with the increase in Ni(II)(DMG)₂ loading, OH⁻, and alcohol concentrations. Rotating disk electrode results obtained with a Ni(II)(DMG)₂ coated graphite disk electrode showed that the electrocatalytic oxidation of alcohol is a 4-electron process producing formate anion (methanol oxidation) or acetate anion (ethanol oxidation). A mechanism for the electrocatalytic oxidation of methanol/ethanol was proposed, and a rate-determining step was also discussed.     

Direct ethanol fuel cells with catalysed metal mesh anodes

Platinum based binary and ternary catalysts prepared by thermal decomposition on titanium mesh were characterised and compared in terms of the electrochemical activity for ethanol oxidation. An enhancement in the catalytic activity was observed for the binary catalyst containing tin and ruthenium in their compositions with platinum. The catalysts were tested in single direct ethanol fuel cells and the result obtained with PtRu and PtSn showed that the mesh based electrodes show competitive performance in comparison to the conventional carbon based anodes.     

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.     

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.     

A study of the electro-catalytic oxidation of methanol on a cobalt hydroxide modified glassy carbon electrode

Cobalt hydroxide modified glassy carbon electrodes (CHM/GC) prepared by the anodic deposition in presence of tartrate ions have been used for the electro-catalytic oxidation of methanol in alkaline solutions where the methods of cyclic voltammetery (CV), chronoamperometry (CA) and impedance spectroscopy (IS) have been employed. In CV studies, in the presence of methanol the peak current of the oxidation of cobalt hydroxide increase is followed by a decrease in the corresponding cathodic current. This suggests that the oxidation of methanol is being catalysed through the mediated electron transfer across the cobalt hydroxide layer comprising of cobalt ions of various valence states. A mechanism based on the electro-chemical generation of Co(IV) active sites and their subsequent consumptions by methanol have been discussed and the corresponding rate law under the control of charge transfer has been developed and kinetic parameters have been derived. In this context the charge transfer resistance accessible both theoretically and through the IS studies have been used as a criteria. Under the CA regimes the reaction followed a Cottrellian behaviour.     

Electrodeposition of Pt-Ru nanoparticles on fibrous carbon substrates in the presence of nonionic surfactant: Application for methanol oxidation

Liquid crystalline and micellar aqueous solutions of the nonionic surfactant Triton X-100 were used to direct the electrodeposition of Pt-Ru nanoparticles onto graphite felts, which were investigated as novel anodes for the direct methanol fuel cell. The effects of surfactant concentration, current density and deposition time in the preparation of these three-dimensional electrodes were studied in a factorial experiment and the electrodes were characterized by SEM and ICP-AES. Cyclic voltammetry, chronoamperometry and chronopotentiometry were carried out to assess the activity of the catalyzed felts for methanol oxidation. The presence of Triton X-100 (40-60 wt.%) coupled with an acidic plating solution were essential for the efficient co-electrodeposition of Ru in the presence of Pt to yield approximately a 1:1 Pt:Ru atomic ratio in the deposit. The highest mass specific activity, 24 A g⁻¹ at 298 K (determined by chronoamperometry after 180 s at 0 V versus Hg/Hg₂SO₄, K₂SO_4std), was obtained for the Pt-Ru electrodeposited in the presence of 40 wt.% Triton X-100 at 60 A m⁻², 298 K for 90 min. Surfactant mediated electrodeposition is a promising method for meso-scale (ca. 10-60 nm diameter) catalyst particle preparation on three-dimensional electrodes.     

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.     

Modeling of the anode side of a direct methanol fuel cell with analytical solutions

In this work, analytical solutions were derived (for any methanol oxidation reaction order) for the profiles of methanol concentration and proton current density, by assuming diffusion mass transport mechanism, Tafel kinetics, and fast proton transport in the anodic catalyst layer of a direct methanol fuel cell. An expression for the Thiele modulus that allows to express the anodic overpotential as a function of the cell current and kinetic and mass transfer parameters was obtained. For high cell current densities, it was found that the Thiele modulus (?²) varies quadratically with cell current density; yielding a simple correlation between anodic overpotential and cell current density. Analytical solutions were derived for the profiles of both local methanol concentration in the catalyst layer and local anodic current density in the catalyst layer. Under the assumptions of the model presented here, in general, the local methanol concentration in the catalyst layer cannot be expressed as an explicit function of the position in the layer. In spite of this, the equations presented here for the anodic overpotential allow the derivation of new semi-empirical equations.     

Electrochemical and spontaneous deposition of ruthenium at platinum electrodes for methanol oxidation: an electrochemical quartz crystal microbalance study

PtRu electrodes with Ru surface concentration ranging from 20 to 50% were prepared by electrolysis of Ru(NO)(NO₃)₃ at a constant potential and/or by spontaneous Ru deposition performed at open circuit potential from a RuCl₃ solution. The amount of either spontaneously or electrochemically deposited ruthenium on the platinum electrode was determined by means of an electrochemical quartz crystal microbalance (EQCM). The effect of the Ru surface concentration on the rate of methanol electrooxidation was also investigated and correlated to the EQCM measurements.     

Effect of graphite felt and activated carbon fiber felt on performance of freshwater sediment microbial fuel cell

BACKGROUND: Sediment microbial fuel cells (SMFCs) could be used as power sources and one type of new technology for the removal of organic matters in sediments. Various types of materials have been used as electrodes. Nevertheless, there is still room to improve electrode materials and enhance their effect on the performance of SMFCs. In this work, performances of SMFCs with activated carbon fiber felt (ACFF) and with nitric acid‐treated ACFF were compared with graphite felt (GF) materials. RESULTS: The maximum power density of the SMFC with ACFF electrode was the highest (33.5 ± 1.5 mW m^?2). Nitric acid‐treated GF electrode slightly increased the maximum power density of SMFC, while the nitric acid treated‐ACFF resulted in significant decline in the maximum power density of SMFC. The maximum power density further increased to 74.5 ± 7.5 mW m^?2 in SMFC using GF cathode and ACFF anode. CONCLUSIONS: ACFF as anode can enhance the transport of electrons from the oxidation of organic matter in the sediment, while the output power was found to reduce in SMFC with ACFF cathode. Further efforts are needed to study the formation conditions of the biocathode and new electrode modification technology. Copyright © 2012 Society of Chemical Industry     

Electrodeposited Pd-Co catalyst for direct methanol fuel cell electrodes: Preparation and characterization

Pd-Co alloy has been recently proposed as a catalyst for the cathode of direct methanol fuel cells with both excellent oxygen reduction activity and methanol tolerance, hence electrodeposition of this alloy is an attractive approach for synthesizing porous metal electrodes with high methanol tolerance in direct methanol fuel cells. In this study, we electrodeposited two types of Pd-Co films onto Au substrates by applying different current density (−10 or −200 mA cm⁻²); and then characterized them in terms of morphology, composition, crystal structure, and catalytic activity. Pd-Co deposited at −10 mA cm⁻² was smooth and possessed smaller particles (ca. 10 nm), while that at −200 mA cm⁻² was dendritic (or rough) and possessed larger particles (ca. 50 nm). Both the Pd-Co alloys were found to be almost the same structure, i.e. a solid solution of ca. Pd₇Co₃ with Pd-skin, and also confirmed to possess comparable activity in oxygen reduction to Pt (potential difference at 1.0 μA cm⁻² was 0.05 V). As for methanol tolerance, cell-voltage was not influenced by addition of 1 mol dm⁻³ methanol to the oxidant solution. Our approach provides fundamental technique for synthesizing Pd-Co porous metal electrodes by electrodeposition.     

Performance of non-porous graphite and titanium-based anodes in microbial fuel cells

Four non-porous materials were compared for their suitability as bio-anode in microbial fuel cells (MFCs). These materials were flat graphite, roughened graphite, Pt-coated titanium, and uncoated titanium. The materials were placed in four identical MFCs, of which the anode compartments were hydraulically connected in series, as well as the cathode compartments. The MFCs were operated with four resistors. The anode kinetics at these electrode materials were studied by means of dc-voltammetry and electrochemical impedance spectroscopy (EIS). Both techniques were compared and showed that the bio-anode performance decreased in the order roughened graphite > Pt-coated titanium > flat graphite > uncoated titanium. Uncoated titanium was unsuitable as anode material. For the other three materials, specific surface area was not the single variable explaining the differences in current density for the different materials. All polarization curves showed a clear limiting current. This limit could not be attributed to mass transfer of the substrate and reflected the maximum biomass activity. The current density of the non-porous bio-anodes, except for the uncoated titanium anode, was comparable to the reported current densities of porous materials when normalized to the projected surface area. The high current densities that were recorded by dc-voltammetry, however, could not be maintained in a stable way for a longer period. This shows that polarization curves of MFCs should be evaluated critically.     

Pt-Co supported on single-walled carbon nanotubes as an anode catalyst for direct methanol fuel cells

Catalyst of Pt-Co supported on single-walled carbon nanotubes (SWCNTs) is prepared using mixed reducing agents. The SWCNTs were pretreated in a microwave oven to enable surface modification. Pt-Co nanoparticles with narrow particle size distribution around 5.4 nm were uniformly deposited onto the SWCNTs. Under same Pt loading mass and experimental conditions, the SWCNTs-Pt-Co catalyst shows higher electrocatalytic activity and improved resistance to CO poisoning than the SWCNTs-Pt catalyst.     

Synergistic effect of RE2O3 (RE = Sm, Eu and Gd) on Pt/mesoporous carbon catalyst for methanol electro-oxidation

This work aims at developing the synergetic catalysis of rare earth oxides, which can serve as the anchoring sites for Pt and in turn as the active sites for the methanol electro-oxidation. Ordered mesoporous carbon has been modified with RE₂O₃ (RE = Sm, Eu and Gd) and subsequently deposited with Pt nanoparticles by a microwave heating process. The catalytic activities for methanol electro-oxidation are evaluated by cyclic voltammogram, chronoamperometry and electrochemical impedance spectroscopy. Pt/C-Sm₂O₃ exhibits an improvement on dispersion and electrocatalytic performance, with the highest mass activity (145.1 mA mg⁻¹) as well as good stability. A possible mechanism is proposed for the effect of RE₂O₃ on Pt nanoparticles. The adsorbed hydroxyl species supplied by RE₂O₃ in the support are contributed to the release of Pt active sites through CO removal and therefore promote the catalytic activity spontaneously.     

Direct methanol alkaline fuel cells with catalysed anion exchange membrane electrodes

Membrane electrodes prepared by chemical deposition of platinum directly onto the anion exchange membrane electrolyte were tested in direct methanol alkaline fuel cells. Data on the cell voltage against current density performance and anode potentials are reported. The relatively low fuel cell performance was probably due to the low active surface area of Pt deposits on the membrane comparing to other membrane electrode assembly (MEA) fabrication methods. However, the catalysed membrane electrode showed good performance for oxygen reduction. A reduction in cell internal resistance was also obtained for the catalysed membrane electrode. By combining the catalysed membrane electrodes with a catalysed mesh, maximum current density of 98 mA cm^–2 and peak power density of 18 mW cm^–2 were achieved.     

Modification of Nafion membranes with ternary composite materials for direct methanol fuel cells

Composite membranes for direct methanol fuel cells (DMFCs) were prepared by using Nafion115 membrane modification with polyvinyl alcohol (PVA), polyimide (PI) and 8-trimethoxysilylpropyl glycerin ether-1,3,6-pyrenetrisulfonic acid (TSPS). The performance of the composite membranes was evaluated in terms of water sorption, dimensional stability, thermal stability, proton conductivity, methanol permeability and cell performance. The proton conductivity was slightly decreased by 1-3% compared with Nafion115, which still kept the high proton conduction of Nafion115. The methanol permeability of Nafion/PI-PVA-TSPS composite membranes was remarkably reduced by 35-55% compared with Nafion115. The power density of DMFCs with Nafion/PI-PVA-TSPS composite membranes reached to 100 mW/cm², exceeding that with Nafion115 (68m W/cm²).     

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.     

The electro-oxidation of carbon monoxide, hydrogen/carbon monoxide and methanol in acid medium on Pt-Sn catalysts for low-temperature fuel cells: A comparative review of the effect of Pt-Sn structural characteristics

The electrocatalytic activity for CO, H₂/CO and CH₃OH oxidation of Pt-Sn catalysts has been extensively investigated for a possible use as anode materials for low-temperature fuel cells. This paper presents an overview of the relationship between the structural characteristics of the catalysts (catalyst composition, degree of alloying, presence of oxides) and their electrocatalytic activity for the oxidation of the different fuels.