Structural designing of Pt-CeO2/CNTs for methanol electro-oxidation

In an attempt to utilize CeO₂ as a co-catalyst with Pt for methanol electro-oxidation, Pt-CeO₂/CNTs were prepared through structural designing by adsorbing Pt nanoparticles on CeO₂ coated CNTs. X-ray Diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) were used to analyze the composition of the prepared catalysts. Zeta potential analysis, high resolution transmission electron microscopy (HRTEM) and cyclic voltammetry (CV) methods indicated that Pt nanoparticles are selectively adsorbed on CNTs other than CeO₂ surface. Pt-CeO₂/CNTs were compared with Pt supported on CNTs in terms of electrochemical active surface (EAS) areas, methanol electro-oxidation activity, and chronoamperometry, results indicating that CeO₂ can enhance the catalytic activity of Pt for methanol electro-oxidation with no apparent decrease of EAS. The CO stripping test showed that CeO₂ can make CO stripped at a lower potential, which is helpful for CO and methanol electro-oxidation.     

A novel co-precipitation method for preparation of Pt-CeO2 composites on multi-walled carbon nanotubes for direct methanol fuel cells

Ceria (CeO₂) as co-catalytic material with Pt on multi-walled carbon nanotubes (Pt-CeO₂/MWCNT) is synthesized by a co-precipitation method. The physicochemical characterizations of the catalysts are carried out by using transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) techniques. Electrocatalytic activities of the catalysts for methanol oxidation is examined by cyclic voltammetry and chronoamperometry techniques and it is found that Pt-CeO₂/MWCNT catalysts exhibited a better activity and stability than did the unmodified Pt/MWCNT catalyst. CO-stripping results indicate the facile removal of intermediate poisoning species CO in the presence of CeO₂, which is helpful for CO and methanol electro-oxidation.     

Preparation and characterization of Pt-CeO2 and Pt-Pd electrocatalysts for the oxygen reduction reaction in the absence and presence of methanol in alkaline medium

In this work, we report the synthesis and characterization of unsupported Pt-CeO₂ (1:1 wt. % Pt:CeO₂ ratio) and Pt-Pd (1:1 wt. % Pt:Pd ratio) electrocatalysts as candidate cathodes for alkaline direct methanol fuel cells (A-DMFCs). The catalytic activity of the cathodes for the oxygen reduction reaction (ORR) in the absence and presence of methanol, in KOH as electrolyte, was evaluated at room temperature. The materials were prepared by chemical reduction with NaBH₄, and pyrolysis at 300 and 600 °C under a H₂/N₂ atmosphere. The XRD results indicated the formation of polycrystalline materials with particle sizes ranging from 9 to 19 nm. Analysis by HRTEM showed the formation of nanostructures with lattice fringes corresponding to Pt, Pd (i.e., the Pt-Pd cathode), or CeO₂ (i.e., the Pt-CeO₂ material). The electrochemical characterization in 0.1 mol L⁻¹ KOH showed that the Pt-Pd is highly active for the ORR in alkaline medium, delivering higher onset potential and mass activity than Pt-alone. Meanwhile, the Pt-CeO₂ material showed slightly lower ORR mass activity than Pt. However, in the presence of methanol, the Pt-CeO₂ nanocatalyst demonstrated significantly higher selectivity and tolerance capability to the alcohol than Pt and Pt-Pd.     

Fundamental understanding of the role of potassium on the activity of Pt/CeO2 for the hydrogen production from ethanol

The effect of potassium as a promoter on the activity of Pt/CeO₂ catalysts for hydrogen production from ethanol was studied in this work. The Pt/CeO₂ catalysts with or without potassium were prepared with incipient impregnation method and analyzed using synchrotron-based X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), oxygen adsorption, ethanol temperature programmed desorption (TPD) NH₃-TPD, and TPR. We find that Pt particles on all CeO₂-based catalysts were less than 2 nm. Potassium addition did not have benefit on hydrogen production activity. However, potassium modified active sites, acidity, and interaction between Pt and CeO₂. Oxygen storage capacity (OSC) was decreased, while the acidity was neutralized and the interaction between Pt and CeO₂ was weakened when potassium was added to Pt/CeO₂. Potassium promoted ethanol decomposition reaction while inhibited decarbonylation of acetaldehyde and CH₄ formation. This work confirms our previous finding that Pt-CeO₂ synergistic interaction played an important role for hydrogen production from bio-alcohols.     

Pt-CeO2/TiN NTs derived from metal organic frameworks as high-performance electrocatalyst for methanol electrooxidation

Metal organic frameworks (MOFs) have attracted tremendous attention in recent years owing to their high-specific surface area (SSA) and variable porous structures. Owing to the strong interaction between Pt and CeO₂, Pt combined steadily with CeO₂. Furthermore, the surface of CeO₂ can activate water to produce hydroxyl groups, which can accelerate the removal of catalytic intermediate CO. But the bad conductivity of metal oxide is still a huge obstacle. More importantly, utilizing TiN with excellent conductivity as support can strengthen conductivity of catalyst and improve catalytic activity. Herein, a novel Pt-CeO₂/TiN Nanotubes (TiN NTs) catalysts derived from Ce-MOF was fabricated for the first time. In the synthesis process of the targeted catalyst, the compounds of Ce-MOF and TiN NTs was prepared via the hydrothermal method and post-nitriding treatment, and implemented as the Pt support. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen adsorption/desorption and electrochemical measurements were carried out to characterize the catalyst. Notably, the peak current density of Pt-CeO₂/TiN NTs (0.67 A mg⁻¹Pt) was approximately 3 times higher than Pt/C (0.28 A mg⁻¹Pt) during methanol oxidation test, showing the exceptional properties toward methanol oxidation reaction (MOR). Remarkably, electrochemical testing data verified the superior tolerance to CO and enhanced catalytical activity of Pt-CeO₂/TiN NTs and it could be attributed to the porous structures and the interaction between TiN NTs and CeO₂.     

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.     

Enhancing hydrogen storage on carbon nanotubes via hybrid chemical etching and Pt decoration employing supercritical carbon dioxide fluid

Acidic etching and Pt particle decoration were applied to modify the hydrogen absorption behavior of carbon nanotubes (CNTs). Two different acidic solutions, namely H₂SO₄/HNO₃ and FeSO₄/H₂SO₄/H₂O₂, were used for etching treatment. A novel electroless deposition process, incorporating supercritical CO₂ (sc-CO₂) fluid, was used to decorate finely-dispersed nano-sized Pt particles on CNTs. The hydrogen storage capacities of various modified CNTs were measured by using a high pressure thermal gravimetric microbalance (HPTGA). The experimental results showed that acidic etching could increase the surface defect density and lead to open-up of the caps of CNTs, resulting in an increase in the active adsorption site for physical sorption of H_2. The electroless deposition of nano-Pt particles on CNTs, using conventional electrolyte, could promote chemical sorption of hydrogen via spillover mechanism. By employing sc-CO₂ bath, the Pt particle size became much finer and more uniformly distributed on the surfaces of CNTs, giving rise to a high hydrogen storage capacity. When a hybrid process including sc-CO₂ Pt decoration following acidic etching was applied to modify CNTs, a substantial enhancement of hydrogen storage capacity (about 2.7 wt%) was observed.     

Cerium oxide-modified surfaces of several carbons as supports for a platinum-based anode electrode for methanol electro-oxidation

Catalyst composites based on Pt and CeO₂ on carbon for methanol oxidation were successively prepared for application in direct-methanol fuel cells (DMFCs). In this work, the catalyst was modified by decoration of CeO₂ onto several carbons, including carbon black (CB), carbon nanotubes (CNT), graphene oxide (GO), reduced graphene oxide (rGO) and mixed carbons, followed by the electrochemical deposition of Pt. The dispersal of CeO₂ and Pt nanoparticles onto the carbon surfaces was confirmed with a face-centred cubic structure. The use of single and mixed carbons takes admirable advantage of the coexisting CeO₂ and Pt nanoparticles, confirming the positive effect of various carbon structures for electrocatalytic enhancement towards methanol oxidation. The CeO₂ also improves the ability for CO oxidation, resulting in a reduction of CO poisoning. The outcomes show an enhancement of the activity and stability so that such alternative as-prepared materials can be introduced to improve the anodic oxidation in DMFCs.     

Electro-catalytic activity of enhanced CO tolerant cerium-promoted Pt/C catalyst for PEM fuel cell anode

Cerium-promoted Pt/C catalysts were prepared by one-pot synthesis process and applied as an anode material for CO tolerance in PEM fuel cell. Its physical properties were characterized by XRD and TEM techniques, which indicated that Pt nano-particles are highly dispersed on the carbon supports. The investigation focused on examining the CO tolerance in sulfur acid solution of Pt–CeO₂/C compared to Pt/C (JM). The hydrogen oxidation activity was strongly depended on the content of the cerium in the Pt catalyst which was detected by CV, LSV, CO-stripping and EIS techniques. Effect of the anode catalyst poisoning on hydrogen oxidation in the presence of CO was studied in single cells. Pt–CeO₂/C catalyst at the appropriate content of 20% Ce presented a very higher CO tolerant activity. A tentative mechanism is proposed for a possible role of a bi-functional synergistic effect between Pt and CeO₂ for the enhanced electro-oxidation of CO. CeO₂-promoted Pt/C catalyst may be one of the attractive candidates as CO tolerance anode material in PEMFC.     

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.     

Effects of Pt precursors on Pt/CeO2 to water-gas shift (WGS) reaction activity with Langmuir-Hinshelwood model-based kinetics

The crystallite size effects of Pt nanoparticles on the CeO₂ (Pt/CeO₂) prepared with four different Pt precursors were investigated in terms of their thermal stability and catalytic activity for a water-gas shift (WGS) reaction using the compositions of reformates after a typical steam reforming of propane. The Pt/CeO₂ prepared with a diamine dinitroplatinum (Pt(NO₂)₂(NH₃)₃) precursor, which forms the cationic Pt(NH₃)₂²⁺ species on the negatively-charged CeO₂ surfaces, revealed a superior catalytic activity and thermal stability by forming the partially oxidized smaller Pt nanoparticles decorated with metallic Pt surfaces as well as by forming the strongly interacted PtOx-CeO₂ interfaces. The stable preservation of the pristine smaller Pt nanoparticles with small aggregations even under the hysteresis test from 250 to 400 °C was mainly attributed to the strong metal-support interactions. The optimized Pt/CeO₂ was further studied to obtain kinetic equations derived by Langmuir-Hinshelwood (LH) model, and the optimal operating conditions of WGS reaction were found to be ~280 °C and H₂O/CO molar ratio of 9 with the activation energy of ~78.4 kJ/mol.     

Atomic layer deposition assisted Pt-SnO2 hybrid catalysts on nitrogen-doped CNTs with enhanced electrocatalytic activities for low temperature fuel cells

Nitrogen doped carbon nanotubes (CN_x) of a high nitrogen concentration were synthesized directly on carbon paper as the skeleton of a 3D composite electrode. Ultra-fine SnO₂ nanoparticles about 1.5 nm were deposited on CN_x with atomic layer deposition (ALD) technique. Pt nanoparticles from 1.5 to 4 nm were deposited on CN_x/carbon paper and SnO₂/CN_x/carbon paper with ethylene glycol reduction method. Three dimensional Pt/CN_x/carbon paper and Pt-SnO₂/CN_x/carbon paper composite electrodes were obtained, respectively. They were characterized over oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) for low temperature fuel cells. With similar sizes of Pt nanoparticles, the electrochemical surface area (ECSA) of Pt-SnO₂/CN_x/carbon paper is larger than that of Pt/CN_x/carbon paper. Pre-deposited SnO₂ nanoparticles promote the electrocatalytic activity of Pt toward ORR, carbon monoxide (CO) stripping and MOR. The underlying mechanisms for the enhanced activities are discussed.     

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.     

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.     

Pt/CNTs-Nafion reinforced and self-humidifying composite membrane for PEMFC applications

A novel thin three-layer reinforced and self-humidifying composite membrane has been developed for PEMFCs. The membrane has two outer layers of plain Nafion and a middle layer of Pt/carbon nanotubes (Pt/CNTs) dispersed Nafion. The Pt/CNTs present in the membrane provides the sites for the catalytic recombination of H₂ and O₂ permeating through the membrane from the anode and cathode to produce water and improve the mechanical properties of the composite membrane at the same time. The water produced directly humidifies the membrane and allows the operation of PEMFCs with dry reactants. The electrochemical performance and mechanical properties of the composite membranes are compared with those of a commercial Nafion^® membrane. The self-humidifying composite membrane could minimize membrane conductivity loss under dry conditions and improve mechanical strength due to the presence of the Pt/CNTs.     

Ni–CeO2 composite cathode material for hydrogen evolution reaction in alkaline electrolyte

In this work, nickel-based electrodes were prepared using composite electrodeposition technique in a nickel sulphamate bath containing suspended micro- or nano-sized CeO₂ particles. The prepared Ni–CeO₂ composite electrodes exhibit an enhanced high catalytic activity toward hydrogen evolution reaction (HER) in alkaline solutions. X-ray diffraction patterns indicated that the CeO₂ particles have been successfully incorporated into the Ni matrix and altered the texture coefficient (TC) of the Ni layer. The morphology of the obtained coatings was characterized by Scanning Electron Microscopy, and the CeO₂ content was determined by coupled energy dispersive X-ray spectrometry. The thermal stability of the composite electrodes was analyzed by thermogravimetric and differential scanning calorimetry, showing a good thermal stability. The catalytic activity of the composite electrodes for HER was measured by steady-state polarization and electrochemical impedance spectroscopy techniques in 1.0 M NaOH solution at room temperature. The exchange current density of HER on the Ni–CeO₂ composite electrodes was much higher than that on Ni electrode. EIS results suggested that a synergetic effect on HER may exist between CeO₂ particles and Ni matrix. Compared to nano-CeO_2, the micro-CeO₂ derived composite electrodes showed higher electrochemical activity. The possible correlation among particle size, content and catalytic activity is discussed.     

Effect of Co in the efficiency of the methanol electrooxidation reaction on carbon supported Pt

The effect of Co addition to carbon nanotubes supported Pt in the methanol oxidation reaction has been investigated by means of differential electrochemical mass spectrometry (DEMS). It has been observed that the CO₂ efficiency increases in carbon nanotubes supported PtCo compared to its homologous Pt catalysts, especially at potentials lower than 0.55 V. Despite of this, the Faradaic current reached by the bimetallic catalysts in the methanol electrooxidation was lower than those recorded on the monometallic samples. This is because Co addition difficult finding enough Pt vicinal sites for methanol dehydrogenation. On the other hand, it has been found that alloying Pt with Co, shifts down the d-band center of the larger element, so the strength of the interaction with adsorbates decreases. Consequently, it will be easier to oxidize CO_ad on the bimetallic surface. Furthermore, the necessary -OH_ad species for the CO_ad oxidation to CO₂ will be provided by the CNTs themselves.     

Solvent effect in the synthesis of nanostructured Pt–Sn/CNT as electrocatalysts for the electrooxidation of ethanol

Pt and Pt–Sn nanoparticles were synthesized and supported onto carbon nanotubes (CNT), the electrocatalytic activity towards the ethanol oxidation reaction was analyzed. The effect of the solvent employed for the synthesis was evaluated. Metal nanoparticles synthesis was made using water (Pt–Sn/CNT-W) or ethanol (Pt–Sn/CNT-E) as a solvent. Pt–Sn/CNT-W material presented only Pt–Sn alloy nanoparticles homogeneously distributed on the carbon nanotubes support. Pt–Sn/CNT-E sample showed non well-dispersed nanoparticles forming agglomerates along the CNTs surface with predominantly Sn⁴⁺ superficial species (SnO₂) as show the XPS, FTIR and electrochemical results. These surface arrangements had important effects on the electrocatalytic properties. Pt–Sn/CNT-W shows higher ethanol electrooxidation activity than the Pt–Sn/CNT-E, which is attributed to: a) the double catalytic effect and the intrinsic electronic mechanism favored by the presence of Sn; b) the good particle dispersion of the bimetallic active phase on the CNT and; c) the absence of SnO₂ species.     

Preparation and photoelectrochemical properties of functional carbon nanotubes and Ti co-doped Fe2O3 thin films

Functional carbon nanotubes (CNTs) were incorporated into Ti-doped Fe₂O₃ thin films by a facile, one-step co-electrodeposition method. The films were characterized by X-ray diffraction, scanning electron microscopy, UV–visible absorption, and X-ray photoelectron spectroscopy. The introduction of CNTs results in a better absorption in visible region and greatly enhances the photoelectrochemical properties of the Ti–Fe₂O₃ films. The improved photoelectrochemical properties of the CNTs and Ti co-doped Fe₂O₃ films are due to the charge equilibration which interplays between the Ti–Fe₂O₃ and CNTs. The effect of CNTs to mediate fast charge transfer and to retard charge recombination rate in the composites is also demonstrated by kinetics analysis and electrochemical impedance spectroscopy. The influence of different groups-modified CNTs and different content of CNTs was also studied. The highest photocurrent is 4.5 mA/cm² at 1.23 V (vs. RHE) obtained by incorporating 0.10 mg/mL amino-group modified CNTs in the Ti–Fe₂O₃ film. The amino-functionalized CNTs doped film exhibits the highest photoelectric response compared with those doped by the pristine and acid-treated CNTs under the same conditions, which can be ascribed to the better hydrophilicity and dispersibility of the amino-functionalized CNTs.     

Simultaneous production of hydrogen and carbon nanotubes from biogas: On the effect of Ce addition to CoMo/MgO catalyst

In this study, hydrogen and carbon nanotubes (CNTs) are simultaneously produced via a synergistic combined process of CO₂ methanation (METH) and chemical vapor deposition (CVD) processes using biogas as a feedstock. METH process could upgrade CO₂ containing biogas into CH₄-rich gas which then decomposed into H₂ and forming CNTs over CoMo/MgO catalyst by CVD process. The effects of Ce addition to CoMo/MgO were investigated. Comprehensive characterization confirms that all as-synthesized samples composed of well-aligned multi-walled carbon nanotubes (MWCNTs) with a narrow size distribution. The Ce addition improved CoMo dispersion on MgO, resulting in smaller and uniform CNTs. The small addition of Ce into CoMo/MgO catalyst could enhance the production CNTs yield. The higher Ce addition would, however, result in the CNTs yield decreased, attributed to a high basicity of CeO₂ surface and a large coverage of CeO₂ on the catalyst surface. The I_G/I_D increased with increased Ce addition, while the surface area monotonically decreased, attributed to a decrease in defects of nanotubes. In addition, this wisely combined process could result in a remarkable 100%CO₂ elimination, while high CH₄ conversion of 90% was obtained. The H₂ production yield could gain more than 30 vol% with respect to H₂ in the feed stream. The H₂ yield and purity in the effluent gas stream were approximately 90%.