Carbon nanotube supported Pt electrodes for methanol oxidation: A comparison between multi- and single-walled carbon nanotubes

This work presents a detailed comparison between multi-walled (MWNT) and single-walled carbon nanotubes (SWNT) in an effort to understand which can be the better candidate of a future supporting carbon material for electrocatalyst in direct methanol fuel cells (DMFC). Pt particles were deposited via electrodeposition on MWNT/Nafion and SWNT/Nafion electrodes to investigate effects of the carbon materials on the physical and electrochemical properties of Pt catalyst. The crystalloid structure, texture (surface area, pore size distribution, and macroscopic morphology), and surface functional groups for MWNT and SWNT were studied using XRD, BET, SEM and XPS techniques. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the electrochemically accessible surface area and charge transfer resistances of the MWNT/Nafion and SWNT/Nafion electrodes. CO stripping voltammograms showed that the onset and peak potentials on Pt-SWNT/Nafion were significantly lower that those on the Pt-MWNT/Nafion catalyst, revealing a higher tolerance to CO poisoning of Pt in Pt-SWNT/Nafion. In methanol electrooxidation reaction, Pt-SWNT/Nafion catalyst was characterized by a significantly higher current density, lower onset potentials and lower charge transfer resistances using CV and EIS analysis. Therefore, SWNT presents many advantages over MWNT and would emerge as an interesting supporting carbon material for fuel cell electrocatalysts. The enhanced electrocatalytic properties were discussed based on the higher utilization and activation of Pt metal on SWNT/Nafion electrode. The remarkable benefits from SWNT were further explained by its higher electrochemically accessible area and easier charge transfer at the electrode/electrolyte interface due to SWNT's sound graphitic crystallinity, richness in oxygen-containing surface functional groups and highly mesoporous 3D structure.     

Development of a self-supported single-wall carbon nanotube-based gas diffusion electrode with spatially well-defined reaction and diffusion layers

This work reports on the development of a solvent-free method for the fabrication of a self-supported single-wall carbon nanotubes electrode, which is based on successive sedimentation of both SWCNT/surfactant and PtRu-SWCNT/surfactant suspensions followed by a thermal treatment at 130 °C. The as-prepared self-supported electrode showed sufficient mechanical strength for half-cell investigation and membrane-electrodes assembly fabrication. By using a Pt catalyst loading of 1 mg cm⁻², the overall thickness of the gas diffusion electrode reached 95 μm. Its electrochemical activity towards methanol oxidation was investigated by means of cyclic voltammetry and current-voltage polarisation measurements under half-cell and direct methanol fuel cell conditions.     

Glycerol oxidation in a microfluidic fuel cell using Pd/C and Pd/MWCNT anodes electrodes

Pd/C and Pd/MWCNT based electro-catalysts were prepared by impregnation and used as anodes for glycerol electro-oxidation in a microfluidic fuel cell. Average particle size and lattice parameters of the catalysts were determined by X-ray diffraction, resulting in 7.5 and 3.5 nm for Pd/C and Pd/MWCNT respectively. The electro-catalytic activity of Pd/C and Pd/MWCNT was investigated in 0.1 M glycerol. The results obtained by electrochemical studies in half cell configuration showed that the onset potential for glycerol oxidation on Pd/MWCNT was characterized by a negative shift ca. 40 mV compared to Pd/C. The maximum power density obtained was 0.51 and 0.7 mW cm⁻² for Pd/C and Pd/MWCNT respectively. These results are comparable with those obtained for a microfluidic fuel cell that uses glucose as fuel. The results of this work not only show that glycerol can be used as fuel in a microfluidic fuel cell but also its performance is similar to that obtained with others fuels.     

Electrochemical properties of LiCoO2/nanocarbon composites fabricated from organic liquids

We fabricated LiCoO₂/carbon composites by forming helical carbon nanomaterials (HCNs) from organic liquids as carbon sources on the surface of LiCoO₂ particles by chemical vapor deposition (CVD) and estimated their electrochemical properties as cathodes of rechargeable lithium cells. By scanning electron microscopy measurements, we observed HCNs called carbon nanotwists formed on the surface of LiCoO₂ particles. X-ray diffraction measurements suggested that acetic acid supplies LiCoO₂/HCNs with less decomposition of LiCoO₂ than C₂H₂ does. However, we found that the electrochemical properties of cells containing the composites were not as good as those of cells containing acetylene black (AB).     

Hierarchy carbon paper for the gas diffusion layer of proton exchange membrane fuel cells

This communication described the fabrication of a hierarchy carbon paper, and its application to the gas diffusion layer (GDL) of proton exchange membrane (PEM) fuel cells. The carbon paper was fabricated by growing carbon nanotubes (CNTs) on carbon fibers via covalently assembling metal nanocatalysts. Surface morphology observation revealed a highly uniform distribution of hydrophobic materials within the carbon paper. The contact angle to water of this carbon paper was not only very large but also particularly even. Polarization measurements verified that the hierarchy carbon paper facilitated the self-humidifying of PEM fuel cells, which could be mainly attributed to its higher hydrophobic property as diagnosed by electrochemical impedance spectroscopy (EIS).     

High aqueous solubility of carboxylated-carbon nanotubes as support for PtRu nanoparticles: Enhanced dispersion and electrocatalytic performance

One great challenge in the development of noble metal nanoparticles (NPs)/carbon nanotubes nanohybrids as fuel cells electrocatalyst is to explore rationally functionalizing CNTs method for dispersion and stability of noble metal NPs catalysts with better electrocatalytic performance. Here we report a facile strategy to fabricate a carboxymethyl chitosan functionalized CNTs (CMC-CNTs) and demonstrate its application as a promising catalyst support material for direct methanol fuel cells. The developed route rationally utilizes the excellent water-solubility and abundant carboxyl (–COOH) functional groups of the CMC-CNTs as a superior supporting material for growing and supporting PtRu NPs. For methanol electrooxidation, the as-prepared PtRu NPs/CMC-CNTs nanohybrid has extremely large electrochemically active surface area (ESA) and exhibits better electrocatalytic activity and stability than PtRu NPs/CNTs catalyst. This provides a facile approach to synthesize CNTs-based nanoelectrocatalysts for high performance energy conversion devices in the future.     

Oxygen evolution in alkali with gas diffusion electrodes

Progresses in the area of the oxygen evolution reaction (OER) are now occurring at a much faster rate relative to few years ago. For this reason, it has been deemed appropriate to present a critical review of the major and most recent contributions towards a fundamental understanding of what determines the OER electrocatalytic properties of a material. Furthermore, the technologies used to produce practical OER electrodes with top activities are assessed and the current benchmarks of performance are identified. Furthermore, results pertaining to our work on Raney–Ni gas diffusion anodes, which have been optimized in thickness and composition, are presented. An addition of 10 wt.% of a Co₃O₄ micrometric powder as co-catalyst (with Raney Ni–Fe) was found to enhance the polarization behavior and performance. The electrode so obtained achieves activities comparable with those of the best electrodes reported in the literature.     

Fabrication of LiCoO2/helical nanocarbon composites and their effect on lithium cell performance

We fabricate LiCoO₂/helical nanocarbon (HCN) composites by forming HCNs on LiCoO₂ on which iron oxides (Fe₂O₃ or Fe₃O₄) are dispersed (LiCoO₂(Fe₂O₃) or LiCoO₂(Fe₃O₄)) as catalysts for HCN formation, and estimate their electrochemical properties. Granular nanocarbons form on LiCoO₂(Fe₂O₃) and LiCoO₂(Fe₃O₄) at 350 °C although HCNs of about 100 nm in diameter form on LiCoO₂(Fe₂O₃) at 450 °C. Transmission electron microscopy and energy dispersive X-ray spectroscopy measurements show that HCNs consist of stacked graphene layers for LiCoO₂(Fe₂O₃)/HCN composites fabricated at 450 °C. On the other hand, several-nm-thick tetragonal layer exists on the LiCoO₂ substrate and amorphous nanocarbons form on the tetragonal layer for LiCoO₂(Fe₂O₃)/HCN and LiCoO₂(Fe₃O₄)/HCN composites fabricated at 350 °C. X-ray diffraction measurements suggest that Fe₂O₃ and Fe₃O₄ do not completely inhibit LiCoO₂ decomposition. Cathodes containing LiCoO₂(Fe₂O₃)/HCN or LiCoO₂(Fe₃O₄)/HCN fabricated at 350 °C improve rate capability of lithium cells. However, this rate capability is not better than that of cathodes containing a mixture of LiCoO₂ and acetylene black.     

Synthesis and electrochemical capacitance of binderless nanocomposite electrodes formed by dispersion of carbon nanotubes and carbon aerogels

Novel nanocomposite carbon aerogel (CAG)-multi-walled carbon nanotubes (MWNT) materials have been synthesized and studied in 5 M KOH for electrochemical capacitor applications. The amount of MWNT in the nanocomposite was varied from 3 to 10 wt%. High specific surface areas ranging between 670 and 710 m² g⁻¹ were obtained as measured by nitrogen gas adsorption method, whereas the average pore diameter ranged between 1 and 4 nm.     

Silver nanoparticle-decorated carbon nanotubes as bifunctional gas-diffusion electrodes for zinc-air batteries

Thin, lightweight, and flexible gas-diffusion electrodes (GDEs) based on freestanding entangled networks of single-walled carbon nanotubes (SWNTs) decorated with Ag nanoparticles (AgNPs) are tested as the air-breathing cathode in a zinc-air battery. The SWNT networks provide a highly porous surface for active oxygen absorption and diffusion. The high conductivity of SWNTs coupled with the catalytic activity of AgNPs for oxygen reduction leads to an improvement in the performance of the zinc-air cell. By modulating the pH value and the reaction time, different sizes of AgNPs are decorated uniformly on the SWNTs, as revealed by transmission electron microscopy and powder X-ray diffraction. AgNPs with sizes of 3-5 nm double the capacity and specific energy of a zinc-air battery as compared with bare SWNTs. The simplified, lightweight architecture shows significant advantages over conventional carbon-based GDEs in terms of weight, thickness and conductivity, and hence may be useful for mobile and portable applications.     

Study of multi-walled carbon nanotubes for lithium-ion battery electrodes

The potential use of multi-walled carbon nanotubes (MWCNTs) produced by Chemical Vapor Deposition (CVD) as conductive agent for electrodes in Li-ion batteries has been investigated. LiNi_0.33Co_0.33Mn_0.33O₂ (NCM) has been chosen as the active material for positive electrodes, and a nano-sized TiO₂-rutile for the negative electrodes. Also the MWCNTs ability of reversibly inserting Li has been characterized. The electrochemical performances of the electrodes are studied by galvanostatic techniques and cyclic voltammetry. In particular the influence of the nanotubes on the rate capability is evaluated. The addition of MWCNTs significantly enhances the rate performances of NCM-based cathodes at all investigated C-rates. The 1 wt.% MWCNTs in TiO₂ rutile-based anodes accounts for an increase in the rate capability when the electrodes are cycled in the potential range 1.0-3.0 V. The range extension to more negative potentials (i.e. 0.1-3.0 V), however, causes a capacity fading especially at higher current rates. The obtained results demonstrate that the addition of MWCNTs to the electrode composition, even in low amounts, enables an increase in both energy and power densities of a Li-ion battery.     

Platinum sonoelectrodeposition on glassy carbon and gas diffusion layer electrodes

The electrodeposition of Pt on glassy carbon (GC) and gas diffusion layer (GDL) surfaces in dilute chloroplatinic acid solutions (10 mM PtCl₄²⁻ in 0.5 M NaCl) was performed potentiodynamically in the absence and presence of ultrasound (20 kHz) at various ultrasonic powers (up to 6 W) respectively and at (313 ± 2) K. In our conditions, it was found that platinum electrodeposition is an irreversible process which requires a substantial overpotential to drive the formation of Pt nuclei on the GC and GDL surfaces; however, under sonication Pt electrodeposition becomes more facile due to lower concentration and nucleation overpotentials and overall currents are significantly increased compared to silent conditions. It was also observed that the specific electrochemical surface area (SECSA) was significantly affected for Pt/GC and Pt/GDL electrodes prepared in the presence of rotation (GC only) and under sonication compared to those prepared under silent conditions. This finding was explained to be due to both larger and agglomerated platinum nanoparticles formed on the GC and GDL surface caused by forced convection. It was also found that ultrasound produced larger Pt nanoparticles on GC electrodes than those on GDL electrodes.     

Determination of transport parameters for multiphase flow in porous gas diffusion electrodes using a capillary network model

The changes of relative permeability and capillary pressure as a function of liquid water phase saturation, two key parameters in two-phase PEMFC models, are investigated using a capillary network model incorporating an invasion percolation algorithm with trapping. The two-dimensional capillary network accounts for capillary dominated drainage and cluster formation. It is shown that relative permeability is constant for low saturation, but follows a power law of saturation for high saturations, with an exponent of about 2.4 that is independent of network size or heterogeneity. An increase of the network size and reduction in heterogeneity tend to reduce the relative permeability, and relative permeabilities of much less then unity are obtained even for saturations as large as 0.8. Capillary pressure on the other hand does not vary with saturation and network size, but is influenced by heterogeneity only. This suggests that regardless of the interface shape and size, the capillaries at the interface maintain a constant average radius causing the capillary pressure to remain constant. It is finally shown that with appropriate scaling and for a given network heterogeneity, the normalized capillary pressure, single-phase permeability and relative permeability can be deduced for other choices of porous medium physical scales without requiring a new set of simulations.     

Electrochemical activity and stability of Pt catalysts on carbon nanotube/carbon paper composite electrodes

This article reports a facile microwave-assisted approach to synthesize Pt catalysts on carbon nanotube (CNT)/carbon paper (CP) composite through catalytic chemical vapor deposition. The Pt deposits, with an average size of 3–5 nm were uniformly coated over the surface of oxidized CNTs. The electrochemical activity and stability of the Pt–CNT/CP electrode were investigated in 1 M H₂SO₄ using cyclic voltammetry (CV) and ac electrochemical impedance spectroscopy. The Pt catalysts showed not only fairly good electrochemical activity (electrochemically active surface area) but also durability after a potential cycling of >1000 cycles. The analysis of ac impedance spectra associated with equivalent circuit revealed that the presence of CNTs significantly reduced both connect and charge transfer resistances, leading to a low equivalent series resistance ˜0.22 Ω. With the aid of CNTs, well-dispersed Pt catalysts enable the reversibly rapid redox kinetic since electron transport efficiently passes through a one-dimensional pathway. Thus, the CNTs do not only serve as carbon support, they also charge transfer media between the Pt catalysts and the gas diffusion layer. The results shed some light on the use of CNT/CP composite, offering a promising tool for evaluating high-performance gas diffusion electrodes.     

Deposition and electrochemical activity of Pt-based bimetallic nanocatalysts on carbon nanotube electrodes

This article reports an approach to prepare bimetallic Pt–M (M = Fe, Co, and Ni) nanoparticles as electrocatalysts and examines their electrochemical activities in 1 M sulfuric acid. The approach consists of chemical oxidation of carbon nanotubes (CNTs), two-step refluxing, and subsequent thermal reduction in hydrogen atmosphere. Three bimetallic pairs of Pt–M catalysts are found to deposit well onto CNT surface, forming Pt–M/CNT composites. The electrochemical behavior of Pt–M/CNT electrodes was investigated in 1 M H₂SO₄ using cyclic voltammetry (CV) and ac electrochemical impedance spectroscopy. The active surface coverage (=electrochemical surface area/geometric surface area) of Pt–M catalysts is significantly enhanced, i.e., Pt–Co (85.1%) > Pt–Ni (80.4%) > Pt–Fe (76.2%) > Pt (26.3%). This enhancement of electrochemical activity can be attributed to the fact that the introduction of Co and Ni may reduce the required potential for water electrolysis and thus the associated carbon oxidation, thereby contributing to hydrogen adsorption. Equivalent circuit analysis indicates that charge transfer resistance accounts for (i) the major proportion of the equivalent serial resistance of Pt–M/CNT electrodes, and (ii) Pt–Co and Pt–Ni catalysts not only improves the electrochemical capacitance but also lowers the equivalent serial resistance. The results shed some light on how use of Pt–M/CNT composite would be a promising electrocatalyst for high-performance fuel cell applications.     

Simulation of nanostructured electrodes for polymer electrolyte membrane fuel cells

Aligned carbon nanotubes (CNTs) with Pt uniformly deposited on them are being considered in fabricating the catalyst layer of polymer electrolyte membrane (PEM) fuel cell electrodes. When coated with a proton conducting polymer (e.g., Nafion) on the Pt/CNTs, each Pt/CNT acts as a nanoelectrode and a collection of such nanoelectrodes constitutes the proposed nanostructured electrodes. Computer modeling was performed for the cathode side, in which both multicomponent and Knudsen diffusion were taken into account. The effect of the nanoelectrode lengths was also studied with catalyst layer thicknesses of 2, 4, 6, and 10 μm. It was observed that shorter lengths produce better electrode performance due to lower diffusion barriers and better catalyst utilization. The effect of spacing between the nanoelectrodes was studied. Simulation results showed the need to have sufficiently large gas pores, i.e., large spacing, for good oxygen transport. However, this is at the cost of obtaining large electrode currents due to reduction of the number of nanoelectrodes per unit geometrical area of the nanostructured electrode. An optimization of the nanostructured electrodes was obtained when the spacing was at about 400 nm that produced the best limiting current density.     

Effect of coexistence of siloxane on production of hydrogen and nanocarbon by methane decomposition using Fe catalyst

Biogas derived from sewage sludge contains CO₂, siloxane, and methane. In this study, the effect of coexistence of siloxane on the production of hydrogen and carbon nanofiber by methane decomposition using iron oxide-alumina catalyst was investigated. The catalyst was reduced by heating in a flow of methane. Siloxane addition to methane caused a catalytic activity at lower temperatures, shortened the induction period prior to the activity, and accelerated catalytic deactivation. Thermal decomposition of siloxane can occur at a lower temperature compared to that of methane. Carbon species formed by the siloxane decomposition may have a higher reducibility than methane does. The reactivity may lead to a carbon deposition at a lower temperature. Coexistence of CO₂ and siloxane can prolong a catalytic lifetime because CO₂ may inhibit the carbon deposition on catalyst to some extent.     

Microwave-assisted polyol synthesis of Pt-Zn electrocatalysts on carbon nanotube electrodes for methanol oxidation

Bimetallic Pt-Zn catalysts with high and stable electrochemical activity towards sulfuric acid and methanol oxidation were synthesized by microwave-assisted polyol (MP) method. A catalytic chemical vapor deposition was used to directly grow multi-layered carbon nanotubes (CNTs) on carbon paper substrate. The as-grown CNT forest serves as a support for the Pt-Zn catalysts having a mean size of 3-5 nm. The catalytic activities of the supported Pt-Zn catalysts toward acid electrolyte and methanol oxidation were examined by cyclic voltammetry test with potential cycling. Experimental results confirmed that two-stage MP synthesis enables the improvement of electrochemical activity, antipoisoning ability and long-term durability of the binary catalyst. This improvement can be attributed to the bifunctional mechanism of the binary catalysts: the Zn content serves as a promoting center for the generation of Zn-OH species, and more Pt sites are thus available for methanol oxidation. Accordingly, the Pt-Zn/CNT catalyst, prepared by the MP approach, displays a potential candidate for fuel cell application due to its easy fabrication (6 min), low cost and no additional reduction process.     

Nanostructured membranes and electrodes with sulfonic acid functionalized carbon nanotubes

Herein we report the covalent functionalization of multiwall carbon nanotubes by grafting sulfanilic acid and their dispersion into sulfonated poly(ether ether ketone). The nanocomposites were explored as an option for tuning the proton and electron conductivity, swelling, water and alcohol permeability aiming at nanostructured membranes and electrodes for application in alcohol or hydrogen fuel cells and other electrochemical devices. The nanocomposites were extensively characterized, by studying their physicochemical and electrochemical properties. They were processed as self-supporting films with high mechanical stability, proton conductivity of 4.47 × 10⁻² S cm⁻¹ at 30 °C and 16.8 × 10⁻² S cm⁻¹ at 80 °C and 100% humidity level, electron conductivity much higher than for the plain polymer. The methanol permeability could be reduced to 1/20, keeping water permeability at reasonable values. The ratio of bound water also increases with increasing content of sulfonated filler, helping in keeping water in the polymer in conditions of low external humidity level.