Methanol oxidation efficiencies on carbon-nanotube-supported platinum and platinum-ruthenium nanoparticles prepared by pulsed electrodeposition

Dense carbon nanotubes (CNTs, 30-50 nm in diameter, 6-8 μm in length) were grown via a thermal chemical vapor deposition process on titanium treated carbon cloths. Catalysts in the form of either nano-scale platinum (Pt) or platinum-ruthenium (Pt-Ru) particles were then deposited on the CNT surfaces by pulse-mode potentiostatic electrodeposition. Surface morphologies of the prepared electrodes were examined by scanning electron microscopy and transmission electron microscopy. Well dispersed catalysts, Pt alone (particle sizes of 7-8 nm) or Pt-Ru (particle sizes of 3-4 nm) nanoparticles, were successfully electrodeposited on the CNT surfaces in citric acid aqueous solutions. In addition, electrochemical characteristics of the specimens were investigated by cyclic voltammetry in argon saturated sulfuric acid aqueous solutions and in mixed sulfuric acid and methanol aqueous solutions. The catalytic activity of the Pt-Ru/CNTs electrode for methanol oxidation was 1038.25 A g⁻¹_Pt in a mixed solution containing 0.5 M sulfuric acid and 1.0 M methanol.     

Enhanced activity of Pt nanoparticle catalysts supported on manganese oxide-carbon nanotubes for ethanol oxidation

Pt nanoparticles deposited on manganese oxide-carbon nanotubes (Pt/MnO_x-CNTs) are prepared by a microwave-assisted polyol method. Their structure characterizations are carried out by Fourier transform infrared spectrometry (FTIR), thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) measurements, indicating that MnO_x nanoparticles cover the surface of CNTs and then Pt nanoparticles are uniformly dispersed on MnO_x-CNTs with the average particle size of about 2.2 nm. Ethanol oxidation peak current on Pt/MnO_x-CNTs (1141.4 mA mg⁻¹ Pt) is 1.82 times higher than that on Pt/CNTs (626.4 mA mg⁻¹ Pt) and 1.28 times higher than that on PtRu/C (JM) (888.6 mA mg⁻¹ Pt). The Pt/MnO_x-CNT catalyst presents not only excellent electrocatalytic activity and very high stability for ethanol oxidation, but also high tolerance to the poisonous carbonaceous intermediates generated during ethanol oxidation compared to Pt/CNT catalyst. This is attributed to the excellent proton conductivity of MnO_x and the synergistic effect between Pt and MnO_x. The optimum mass ratio of MnO_x to CNTs is 1:1 in the Pt/MnO_x-CNT catalysts.     

Effect of Pt loading on the hydrogen production of CNT/Pt composites functionalized with carboxylic groups

This work reports the morphological and photocatalytic hydrogen generation properties of CNT/Pt composites with and without functionalization by carboxylic/oxygen groups. The composites with and without functionalization were named f-CNT/Pt and CNT/Pt, respectively. Several f-CNT/Pt and CNT/Pt composites with different content of Pt NPs (from 0 to 30 wt%) were synthesized and analyzed by scanning electron microscopy (SEM). Those images revealed that the composites without functionalization presented higher agglomerations of Pt nanoparticles (NPs). Furthermore, the average sizes of the Pt NPs in the named f-CNT/Pt composites (2.3–2.9 nm) were lower than these in the CNT/Pt composites (2.5–3.1 nm). The hydrogen generation rates were also calculated from the decomposition of pure water under UV irradiation (365 nm) and found maximum values of 45.4 and 193.9 μmol·h⁻¹ g⁻¹ for the CNT/Pt and f-CNT/Pt composites (they contained 20 wt% of Pt NPs), respectively. Additional experiments for hydrogen generation were achieved using sodium sulfite as sacrificial agent; in this case, a maximum value of 13850 μmol·h⁻¹ g⁻¹ was obtained for the f-CNT/Pt composite. The f-CNT/Pt composites produced more hydrogen than the CNT/Pt composites because they presented higher content of defects; this was confirmed by the Raman spectra. We also showed that the Pt NPs acted as electron trap centers, which delayed the recombination of the photogenerated electrons and holes, this in turn, enhanced the hydrogen generation rates of the composites (the hydrogen generation was maximized by varying the content of Pt NPs deposited on the CNTs). The CNT/Pt composites presented here were simpler and easier to synthesize than the previous published ternary systems based on TiO₂, CNTs and Pt NPs.     

High loading and monodispersed Pt nanoparticles on multiwalled carbon nanotubes for high performance proton exchange membrane fuel cells

Composite electrodes consisting of Pt nanoparticles-supported on multiwalled carbon nanotubes grown directly on carbon paper (Pt/CNTs/carbon paper) have been synthesized by a new method using glacial acetic acid as a reducing agent. Transmission electron microscopy (TEM) images show that the Pt nanoparticles with high density and relative small in size (2–4 nm) were monodispersed on the surface of CNTs. X-ray photoelectron spectroscopy (XPS) analysis indicates that the glacial acetic acid acts as a reducing agent and has the capability of producing a high density of oxygen-containing functional groups on the surface of CNTs that leads to high density and monodispersion of Pt nanoparticles. Compared with standard Pt/C electrode, the Pt/CNT/carbon paper composite electrodes exhibit higher electrocatalytic activity for methanol oxidation reaction and higher single-cell performance in a H₂/O₂ fuel cell.     

Electrochemical properties of manganese oxide coated onto carbon nanotubes for energy-storage applications

Birnessite-type manganese dioxide (MnO₂) is coated uniformly on carbon nanotubes (CNTs) by employing a spontaneous direct redox reaction between the CNTs and permanganate ions (MnO₄⁻). The initial specific capacitance of the MnO₂/CNT nanocomposite in an organic electrolyte at a large current density of 1 A g⁻¹ is 250 F g⁻¹. This is equivalent to 139 mAh g⁻¹ based on the total weight of the electrode material that includes the electroactive material, conducting agent and binder. The specific capacitance of the MnO₂ in the MnO₂/CNT nanocomposite is as high as 580 F g⁻¹ (320 mAh g⁻¹), indicating excellent electrochemical utilization of the MnO₂. The addition of CNTs as a conducting agent improves the high-rate capability of the MnO₂/CNT nanocomposite considerably. The in situ X-ray absorption near-edge structure (XANES) shows improvement in the structural and electrochemical reversibility of the MnO₂/CNT nanocomposite after heat-treatment.     

Carbon nanotube-supported Pt-HxMoO3 as electrocatalyst for methanol oxidation

Carbon nanotube (CNT)-supported platinum modified with H_xMoO₃ (Pt-H_xMoO₃/CNT) was prepared and used as an electrocatalyst for methanol oxidation. In the preparation of this electrocatalyst, a platinum precursor was loaded on CNTs and reduced by sodium borohydride in ethylene glycol, resulting in CNT-supported platinum without modification (Pt/CNT), and then the Pt/CNT was modified with H_xMoO₃ that was formed by hydrolysis and subsequent reduction of ammonium molybdate. The surface morphology, structure and composition of Pt-H_xMoO₃/CNT and Pt/CNT as well as their activity toward methanol oxidation were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive spectrometry (EDS), Fourier transform infrared spectroscopy (FTIR), cyclic voltammetry (CV), chronoamperometry (CA), chronopentiometry (CP), and electrochemical impedance spectroscopy (EIS). The results, obtained from TEM, XRD, EDS, and FTIR, indicate that the platinum loaded on CNTs has a face-centered cubic structure with particle sizes of 2–5 nm, and the modification of H_xMoO₃ on platinum with an atom ratio of Pt:Mo = 2:1 has little effect on the particle size, distribution and structure of the platinum. The results, obtained from CV, CA, CP, and EIS, show that the Pt-H_xMoO₃/CNT exhibits higher electrocatalytic activity toward methanol oxidation and better carbon monoxide tolerance than Pt/CNT.     

Pd nanoparticles on mesoporous tungsten carbide as a non-Pt electrocatalyst for methanol electrooxidation reaction in alkaline solution

We demonstrate Pd nanoparticles on well-defined mesoporous tungsten carbide (Pd/meso-WC) for methanol electrooxidation in alkaline solution. The meso-WC exhibits mesoporous structure with ∼8.5 nm in average pore size and ∼47 m² g⁻¹ in specific surface area. The Pd nanoparticles with size of ∼3.3 nm are highly dispersed on the meso-WC. The electron transfer from W to Pd due to the difference of electronegativity is confirmed by X-ray photoelectron spectroscopy. The improved electrocatalytic activity and stability for methanol electrooxidation of Pd/meso-WC is likely to be mainly attributed to a strong interaction between Pd nanoparticles and mesoporous tungsten carbide structure.     

Pt and Pt–Ag nanoparticles supported on carbon nanotubes (CNT) for oxygen reduction reaction in alkaline medium

In this work, we investigated the effect of the carbon nanotubes (CNT) as alternative support of cathodes for oxygen reduction reaction (ORR) in alkaline medium. The Pt and Pt–Ag nanomaterials supported on CNT were synthesized by sonochemical method. The crystalline structure, morphology, particle size, dispersion, specific surface area, and composition were investigated by XRD, SEM-EDS, TEM, HR-TEM, N₂ adsorption-desorption and XPS characterization. The electrochemical activity for ORR was evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) in alkaline medium. The electrochemical stability was researched by an accelerated degradation test (ADT). Pt/CNT showed the better electrocatalytic activity towards ORR compared with Pt–Ag/CNT and Pt/C. Pt/CNT exhibited higher specific activity (1.12 mA cm^?2 _Pt) than Pt/C (0.25 mA cm^?2 _Pt) which can be attributed to smaller particle size, Pt-CNT interaction, and Pt load (5 wt%). The Pt monometallic samples supported on CNT and Vulcan showed higher electrochemical stability after ADT than Pt–Ag bimetallic. The ORR activity of all materials synthesized proceeded through a four-electron pathway. Furthermore, the EIS results showed that Pt/CNT exhibited the lower resistance to the transfer electron compared with conventional Pt/C and Pt–Ag/CNT.     

Carbon nanotube modified Zn0.83Cd0.17S nanocomposite photocatalyst and its hydrogen production under visible-light

Visible light photocatalytic H₂ production from water splitting has received much attention for its potential application in converting solar energy into chemical energy. In this paper, carbon nanotube modified Zn_0.83Cd_0.17S nanocomposite was prepared by a solvothermal method. CNTS can efficiently suppress the growth of chalcogenide nanoparticles and improve the dispersity of the nanocomposite. The absorption edges of Zn_0.83Cd_0.17S/CNTs nanocomposites red-shift and the response of the visible-light region (500–800 nm) is strengthened with the increase of CNTs contents in the samples. The prepared Zn_0.83Cd_0.17S/CNTs nanocomposites exhibit an enhanced photocatalytic H₂-production activity and an optimum amount of CNT is determined to be ca. 0.25 wt%, at which the Zn_0.83Cd_0.17S/CNTs displays the highest photocatalytic activity under the irradiation of Xe lamp, with an H₂ production rate of 5.41 mmol h⁻¹ g⁻¹. Furthermore, the prepared Zn_0.83Cd_0.17S/CNTs nanocomposite is photostable and no photocorrosion was observed after photocatalytic recycling, compared with pure Zn_0.83Cd_0.17S photocatalyst.     

Hydrothermal synthesis of Pt/MWCNTs nanocomposite electrocatalysts for proton exchange membrane fuel cell systems

A hydrothermal method for preparation of size-controlled Pt nanoparticles dispersed highly on multiwalled carbon nanotubes (Pt/MWCNTs) has been studied to optimize the effective parameters (temperature, time, pH and stirring rate) using Taguchi method. The synthesized Pt/MWCNTs nanocomposite samples were characterized through X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray fluorescence (XRF) analyses to identify mean Pt nanoparticles size and Pt content. The analysis of the primary experimental data and demonstration of the main effect trend of each parameter showed that a reaction temperature of about 140 °C, a reaction period of 5 h, a slightly basic reaction pH (∼9) and a stirring rate of 500 rpm are the optimum process conditions which give a low mean Pt nanoparticles size (2.8 nm) and a high Pt content (19.4 wt.%) simultaneously. Cyclic voltammetry (CV) analysis showed that under optimum conditions the synthesized sample gives a specific surface area of 99 m² g⁻¹. Obtaining the polarization curves for the two fabricated membrane electrode assemblies (MEAs) using the optimized catalyst and a commercial Pt/C catalyst (10 wt.%, Aldrich) with Pt loading of 0.4 mg cm⁻² demonstrated that the catalyst prepared under optimum conditions shows a considerably better performance.     

High performance of low electrocatalysts loading on CNT directly grown on carbon cloth for DMFC

This study demonstrated the feasibility of a high-performance membrane-electrode-assembly (MEA), with low electrocatalyst loading on carbon nanotubes (CNTs) grown directly on carbon cloth as an anode. The direct growth of CNTs was synthesized by microwave plasma-enhanced chemical vapor deposition using CH₄/H₂/N₂ as precursors. The cyclic voltammetry and electrochemical impedance measurements with 1 mM Fe(CN)₆^3−/4− redox reaction reveal a fast electron transport and a low resistance of charge transfer on the direct growth of CNT. The electrocatalysts, platinum and ruthenium, were coated on CNTs by sputtering to form Pt-Ru/CNTs-CC with carbon cloth for CC. Pt-Ru electrocatalysts are uniformly dispersed on the CNT, as indicated by high-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (TEM), because the nitrogen doped in the CNT acts as active sites for capturing electrocatalysts. The MEA, the sandwiched structure which comprises 0.4 mg cm⁻² Pt-Ru/CNTs-CC as the anode, 3.0 mg cm⁻² Pt black as the cathode and Nafion 117 membrane at the center, performs very well in a direct methanol fuel cell (DMFC) test. The micro-structural MEA analysis shows that the thin electrocatalyst layer is uniform, with good interfacial continuity between membrane and the gas diffusion layer.     

High Pt utilization catalyst prepared by ion exchange method for direct methanol fuel cells

A Pt-Nafion/C catalyst is synthesized through a modified ion-exchange method, in which [Pt(NH₃)₂]²⁺ cations exchange with H⁺ ions of Nafion/C composites and are reduced by H₂. Based on the results from transmission electron microscopy and inductively coupled plasma atomic emission spectrometry, it is found that the average size of Pt particles is around 2.3 nm and the Pt loading is about 1.8 wt% in the Pt-Nafion/C catalyst. Although the Pt mass activities (MAs) toward oxygen reduction reaction tested in half cells are similar, when used as cathode catalyst for a direct methanol fuel cell, the Pt MA as high as 4.36 A mg_Pt⁻¹ are achieved for the Pt-Nafion/C catalyst at 825 mV, which is 6 times higher than that of the Pt/C catalyst. The enhanced Pt utilization could be attributed to the defined location of Pt nanoparticles in the triple phase zones in electrode.     

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.     

Roles of Pb and MnOx in PtPb/MnOx-CNTs catalyst for methanol electro-oxidation

Design of novel nano-scale catalysts with high activity and low cost for methanol oxidation reaction is crucial for the development of direct methanol fuel cell. In this study, MnO_x, Pt and Pb were forced to precipitate successively on the surface of carbon nanotubes for fabricating a PtPb/MnO_x-CNTs catalyst. Physical characterizations indicated that there existed a mass of Mn (IV, Ⅴ), Pb (Ⅱ) and Pt (0) species, and partial alloying between Pt and Pb in this catalyst. Methanol oxidation reaction with this novel composite exhibited over 3 times higher specific activity (140.9 mA cm⁻²) and somewhat lower onset potential (−0.1 V vs. Hg/Hg₂SO₄) than the values on Pt/CNTs (44.2 mA cm⁻² and 0 V, respectively). Fundamental understanding in reaction mechanisms enabled us to reveal the distinguishing functions between Pb and MnO_x in methanol oxidation processes. The addition of Pb resulted in the enhanced intrinsic activity towards electro-oxidation of residual intermediate species, while dehydrogenation in methanol oxidation processes was obviously improved by using MnO_x-CNTs as a support.     

Precise control of π-conjugated polymer/carbon nanotubes heterointerfaces for oxygen reduction reactions

Nitrogen-coordinated metal catalyst has been regarded as a promising candidate for precious platinum for oxygen reduction reaction (ORR). However, controlling the structure and composition of coordinated metals in heterogeneous catalysts remains a synthetic bottleneck. Here, we design and fabricate π-conjugated polymer/CNTs heterointerfaces by polymerizing Co-BTA on CNTs. Co-BTA contains abundant Co–N₄ moieties and provides catalytic sites for ORR. CNT acts as a support and constructs the network for electron transport. Therefore, Co-BTA/CNT exhibits outstanding catalytic activity for ORR with comparable half-wave potential to commercial Pt/C. Furthermore, Co-BTA/CNT demonstrates better durability and methanol tolerance compared with Pt/C. Importantly, zinc-air batteries with Co-BTA/CNT have a maximum discharge power of 94.5 mW cm⁻² and a high energy density of 985 Wh kg⁻¹, superior to that with commercial Pt/C (51.5  mW cm⁻², 930 Wh kg⁻¹). This work paves a new avenue for precisely controlling nitrogen-coordinated metal catalysts for electrochemical energy conversion and storage.     

Preparation of graphene nanosheet/carbon nanotube/polyaniline composite as electrode material for supercapacitors

Graphene nanosheet/carbon nanotube/polyaniline (GNS/CNT/PANI) composite is synthesized via in situ polymerization. GNS/CNT/PANI composite exhibits the specific capacitance of 1035 F g⁻¹ (1 mV s⁻¹) in 6 M of KOH, which is a little lower than GNS/PANI composite (1046 F g⁻¹), but much higher than pure PANI (115 F g⁻¹) and CNT/PANI composite (780 F g⁻¹). Though a small amount of CNTs (1 wt.%) is added into GNS, the cycle stability of GNS/CNT/PANI composite is greatly improved due to the maintenance of highly conductive path as well as mechanical strength of the electrode during doping/dedoping processes. After 1000 cycles, the capacitance decreases only 6% of initial capacitance compared to 52% and 67% for GNS/PANI and CNT/PANI composites.     

Graphene nanosheet–CNT hybrid nanostructure electrode for a proton exchange membrane fuel cell

This work demonstrates two-step growth of graphene nanosheets (GNS), in which carbon nanotubes (CNTs) are grown directly on a carbon cloth. GNS are subsequently constructed on the CNT surface, revealing the stand-up structure of the GNS–CNT hybrid nanostructure. The GNS–CNT hybrid nanostructure shows Nernstian and fast electron-transfer kinetics for electrochemical reactions of Fe(CN)₆^3−/4−. A 0.1 mg cm⁻² Pt/GNS–CNT is used in the cathode of a proton membrane exchange fuel cell, in which the maximum power density is 1072 mW cm⁻² at 80 °C under H₂/O₂. In addition to a low-resistance electron-transfer pathway, the GNS–CNT hybrid nanostructure also provides numerous edge planes with strong electrochemical activity, ultimately enhancing electrochemical activity and fuel cell performance.     

Design and preparation of highly active carbon nanotube-supported sulfated TiO2 and platinum catalysts for methanol electrooxidation

A novel electrocatalyst structure of carbon nanotube-supported sulfated TiO₂ and Pt (Pt-S-TiO₂/CNT) is reported. The Pt-S-TiO₂/CNT catalysts are prepared by a combination of improved sol-gel and ethylene glycol reduction methods. Transmission electron microscopy and X-ray diffraction show that the sulfated TiO₂ is amorphous and is coated uniformly on the surface of the CNTs. Pt nanoparticles of about 3.6 nm in size are homogenously dispersed on the sulfated TiO₂ surface. Fourier transform infrared spectroscopy analysis proves that the CNT surfaces are modified with sulfated TiO₂ and a high concentration of SO_x, and adsorbed OH species exist on the surface of the sulfated TiO₂. Electrochemical studies are carried out using chronoamperometry, cyclic voltammetry, CO stripping voltammetry and impedance spectroscopy. The results indicate that Pt-S-TiO₂/CNT catalysts have much higher catalytic activity and CO tolerance for methanol electrooxidation than Pt/TiO₂/CNTs, Pt/CNTs and commercial Pt/C.     

Preparation of PEDOT-modified double-layered hollow carbon spheres as Pt catalyst support for methanol oxidation

In this paper, Pt nanoparticles (Pt NPs) deposited hybrid carbon support is prepared by modifying double-layered hollow carbon spheres（DLHCs）with poly(3,4-ethylenedioxythiophene) (PEDOT) and used as anode catalyst of methanol oxidation. The structure of nanocomposites is characterized by SEM, TEM, FT-IR, XRD and XPS, confirming the greatly enhanced synergistic effect between the PEDOT and DLHCs, and illustrating the uniform distribution of Pt NPs on the PEDOT/DLHCs composite surface with a small particle size (~2.63 nm). Cyclic voltammetry, chronoamperometry and impedance spectroscopy applied to determine the electrocatalytic activity of catalysts, it is found that the synthesized PEDOT/DLHCs/Pt possesses excellent characteristics such as large electrochemically active surface area and high mass activity of 59.45 m² g⁻¹ and 807 mA mg⁻¹ in 0.5 M H₂SO₄ containing 1 M methanol solution, which is almost 1.24 and 2.8 times greater than those of commercial Pt/C, and the catalyst exhibits superior stability after 500 durability cycles. The enhanced electrocatalytic behavior can be ascribed to the excellent electronic conductivity of PEDOT-modified DLHCs and the strong binding of PEDOT/DLHCs to Pt NPs, suggesting that the PEDOT/DLHCs/Pt is a promising electrocatalyst for direct methanol fuel cell.     

Promoting hydrogen production by loading PdO and Pt on N–TiO2 under visible light

Nitrogen/titanium dioxide (N/TiO₂) visible light photocatalysts were prepared using the sol–gel method. The catalysts were characterized using transmission electron microscopy, reflective UV–visible spectroscopy, specific surface area measurements, and X-ray diffraction. The prepared catalysts were used to generate hydrogen gas through the water-splitting reaction under visible light (wavelengths greater than 400 nm). Various N/Ti addition ratios were tested, and the hydrogen generation rates were compared to determine the optimal ratio. The maximal hydrogen production rate (approximately 55 μmol h⁻¹ g⁻¹) was attained when the N/Ti ratio of N–TiO₂ was 10. When PdO and Pt were loaded onto the N–TiO₂ catalyst, the hydrogen generation rates increased to 544 and 772 μmol h⁻¹ g⁻¹, respectively. The highest hydrogen production rate (2460 μmol h⁻¹ g⁻¹) was obtained when bimetallic 0.05 wt% PdO-0.10 wt% Pt/N–TiO₂ was used. After three times use the hydrogen yield of the catalyst was maintained as 83%. A possible mechanism of water splitting catalyzed by this visible light photocatalyst is proposed.