Methanol electro-oxidation on Ni@Pd core-shell nanoparticles supported on multi-walled carbon nanotubes in alkaline media

A novel method to prepare well-dispersed Ni@Pd core-shell nanoparticles on multi-walled carbon nanotubes (Ni@Pd/MWCNTs) is reported. The morphology and crystallinity of the catalyst are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analyses, respectively. Binary composite Ni@Pd/MWCNTs have been obtained and investigated for electrocatalysis of methanol oxidation in 0.5 M NaOH. It is observed that Ni@Pd/MWCNTs increases the apparent electrocatalytic activity and stability of the electrode considerably than that of PdNi/MWCNTs and Pd/MWCNTs catalysts. It is implied that Ni@Pd core-shell nanoparticles supported on MWCNTs is very promising for portable applications in DMFC in alkaline solution.     

Enhanced electrocatalytic performance for methanol oxidation of Pt nanoparticles on Mn3O4-modified multi-walled carbon nanotubes

We report the preparation of well-dispersed Pt nanoparticles depositing on Mn₃O₄-modified multi-walled carbon nanotubes (Mn₃O₄-MWCNTs) that can be taken as high performance catalyst in methanol electro-oxidation. Various spectrometry techniques such as FT-IR, Raman, XRD, TEM and XPS measurements were performed, revealing that the Pt nanoparticles were highly dispersed on the surface of Mn₃O₄-modified MWCNTs with a narrow size distribution between 1.7 nm and 3.9 nm. Compared the Pt/MWCNT catalyst without Mn₃O₄ modification, the Pt/Mn₃O₄-MWCNT composite catalyst not only shows relative large electrochemical active surface area (EAS), high catalyzing activity toward methanol electro-oxidation, but also exhibits very high stability with apparent anti-poisoning tolerance to the incomplete oxidized species during methanol oxidation.     

Novel Pd/β-MnO2 nanotubes composites as catalysts for methanol oxidation in alkaline solution

In this work, we demonstrated a completely new, simple and effective strategy for preparing catalysts by using β-MnO₂ nanotubes as the supporting materials, and the Pd nanoparticles were coated onto β-MnO₂ nanotubes through a simple reductive process firstly. The as-prepared materials were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electrochemical measurements. The results indicated that the Pd nanoparticles were homogeneously dispersed and well separated from one another on the β-MnO₂ nanotubes surfaces, which makes it have a potential application in catalysts. In this study, we mainly tested the electrochemical performance of Pd/β-MnO₂ for methanol oxidation in alkaline solution. Further research to optimize the synthesis condition, particularly to develop β-MnO₂ nanotubes as supporting materials of other noble metal catalysts is currently in progress.     

Enhanced electrochemical hydrogen storage capacity of multi-walled carbon nanotubes by TiO2 decoration

Electrochemical hydrogen storage of multi-walled carbon nanotubes (MWCNTs) decorated by TiO₂ nanoparticles (NPs) has been studied by the galvanostatic charge and discharge method. The TiO₂ NPs are deposited on the surface of MWCNTs by sol-gel method. Structural and morphological characterizations have been carried out using XRD, SEM and TEM, respectively. TiO₂ NPs can significantly enhance the discharge capacity of MWCNTs. The cyclic voltammograms analysis indicates that the electrical double layer contributes little to the discharge capacity of TiO₂-decorated MWCNTs. The MWCNTs modified with a certain amount of TiO₂ NPs have a discharge capacity of 540 mAh/g, corresponding to an electrochemical hydrogen storage capacity of about 2.02 wt%, which is quite interesting for the battery applications. The enhancement effect of TiO₂ NPs on the discharge capacity of MWCNTs could be related to the increased effective area for the adsorption of hydrogen atoms in the presence of TiO₂ NPs on MWCNTs and the preferable redox ability of TiO₂ NPs.     

A simple approach towards sulfonated multi-walled carbon nanotubes supported by Pd catalysts for methanol electro-oxidation

Functionalized benzenesulfonic groups were grafted onto the surface of multi-walled carbon nanotubes (MWCNTs) supported Pd catalysts in direct methanol fuel cells by a new and simple in situ radical polymerization of 4-styrenesulfonate and isoamyl nitrite. The resultant sulfonated MWCNTs-supported Pd catalysts (S-MWCNTs/Pd) were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectrometry measurements. Electrochemical characterizations of S-MWCNTs/Pd catalysts for methanol electro-oxidation in alkaline solution were investigated by cyclic voltammetry techniques. These results showed that S-MWCNTs/Pd exhibited higher electrocatalytic activity, enhanced CO tolerance and better stable life than did that with the unsulfonated counterparts, mainly due to the easier access with methanol and well dispersed distribution of the S-MWCNTs/Pd catalysts in water. In addition, compared with traditional sulfonation of MWCNTs, this new approach is more advantageous to make small and uniform dispersion of Pd particles loaded onto the surfaces of sulfonated MWCNTs, indicating it is a simple, rapid, and efficient method to functionalize MWCNTs.     

A facile and novel approach toward synthetic polypyrrole oligomers functionalization of multi-walled carbon nanotubes as PtRu catalyst support for methanol electro-oxidation

A new process to prepare well-dispersed PtRu nanoparticles on polypyrrole oligomers (PPO)-functionalized multi-walled carbon nanotubes (PtRu/PPO-MWCNTs) is reported. In this method, bimetallic PtRu electrocatalysts are deposited onto polypyrrole oligomers (PPO)-functionalized MWCNTs by polyol process. The noncovalent functionalization of MWCNTs by PPO is simple and can be carried out at room temperature without the use of expensive chemicals or corrosive acids, thus preserving the integrity and the electronic structure of MWCNTs. PtRu electrocatalysts on PPO-functionalized MWCNTs show much better distribution with no formation of aggregates, higher electrochemically active surface area, and higher electrocatalytic activity and stability for the electro-oxidation of methanol in direct methanol fuel cells as compared to those on conventional acid-treated MWCNTs and carbon black supported PtRu electrocatalysts. It is implied in the study that the method of PPO-functionalized MWCNTs with Pt-based catalysts is promising and will be potential in design and fabrication of electrocatalysis.     

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.     

Long-term cycling behavior of asymmetric activated carbon/MnO2 aqueous electrochemical supercapacitor

Activated carbon–MnO₂ hybrid electrochemical supercapacitor cells have been assembled and characterized in K₂SO₄ aqueous media. A laboratory cell achieved 195,000 cycles with stable performance. The maximal cell voltage was 2 V associated with 21 ± 2 F g⁻¹ of total composite electrode materials (including activated carbon and MnO₂, binder and conductive additive) and an equivalent serie resistance (ESR) below 1.3 Ω cm². Long-life cycling was achieved by removing dissolved oxygen from the electrolyte, which limits the corrosion of current collectors. Scaling up has been realized by assembling several electrodes in parallel to build a prismatic cell. A stable capacity of 380 F and a cell voltage of 2 V were maintained over 600 cycles. These encouraging results show the interest of developing such devices, including non-toxic and safer components as compared to the current organic-based devices.     

Electrocatalysis of carbon black- or chitosan-functionalized activated carbon nanotubes-supported Pd with a small amount of La2O3 towards methanol oxidation in alkaline media

The Pd/C catalysts with and without a small amount of La₂O₃ were synthesized by a simple reduction reaction with sodium borohydride in aqueous solution. The structure and morphology of these catalysts were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy and transmission electron microscopy. The electrocatalytic performance of these catalysts for methanol oxidation in alkaline media was investigated using cyclic voltammetry, chronoamperometry and CO stripping experiments. The results show that the Pd–La₂O₃/C catalyst has a higher catalytic activity than the Pd/C catalyst, but the effect of La₂O₃ cannot be explained by a bi-functional mechanism. X-Ray photoelectron spectroscopy analyses suggest that the higher content of metallic Pd caused by the addition of La₂O₃ contributes to the better catalytic activity of Pd–La₂O₃/C. Based on the good electrocatalytic performance of Pd–La₂O₃/C, the Pd–La₂O₃ catalyst supported on chitosan (CS)-functionalized activated carbon nanotubes was prepared, and it exhibited a better catalytic activity. The improvement is attributed to the good dispersion status of metal particles and the further increase of metallic Pd due to the presence of CS.     

Preparation and characterization of multi-walled carbon nanotube/PBNPI nanocomposite membrane for H2/CH4 separation

By combining organic polymers normally used to make membrane filters with inorganic substances, multi-walled carbon nanotube (MWCNTs), an extraordinary ability to separate H₂ from CH₄ was developed in this study. A series of MWCNTs/PBNPI nanocomposite membrane with a nominal MWCNTs content between 1 and 15 wt% were prepared by solution casting method, in which the very fine MWCNTs were embedded into glassy polymer membrane. Detailed characterizations, such as morphology, thermal stability and crystalline structure have been conducted to understand the structures, composition and properties of nanocomposite membranes. The results found that this new class of membrane had increased permeability and enhanced selectivity, and a useful ability to filter gases and organic vapours at the molecular level.     

Synthesis of Pd/TiO2 nanotubes/Ti for oxygen reduction reaction in acidic solution

Highly ordered and uniformly distributed TiO₂ nanotubes on a pure titanium substrate (TNTs/Ti) are successfully fabricated by a pulse anodic oxidation method as the support for Pd electrocatalyst. Pd is electrochemically deposited onto TNTs/Ti support. The sensitization with SnCl₂ and activation with PdCl₂ are critical for the formation of highly dispersed Pd nanoparticles on the TNTs/Ti support. It has been found that both Pd/TNTs/Ti and Pt electrodes show the similar electrochemical behavior in H₂SO₄, implying the possibility to develop the Pt-free alternative electrocatalyst based on the Pd/TNTs/Ti system in acid medium. The preliminary results in this work show that the Pd/TNTs/Ti catalysts have an acceptable catalytic activity for the oxygen reduction reaction (ORR) in acid medium. The factors influencing the structure of TNTs and the catalytic activity of Pd/TNTs/Ti for the ORR are also studied in detail.     

Hybrid supercapacitor based on MnO2 and columned FeOOH using Li2SO4 electrolyte solution

The nano-sized columned β-FeOOH was prepared by the hydrolysis process and its electrochemical capacitance performance was evaluated for the first time in Li₂SO₄ solution. A hybrid supercapacitor based on MnO₂ positive electrode and FeOOH negative electrode in Li₂SO₄ electrolyte solution was designed. The electrochemical tests demonstrated that the hybrid supercapacitor has a energy density of 12 Wh kg⁻¹ and a power density of 3700 W kg⁻¹ based on the total weight of the electrode active materials with a voltage range 0–1.85 V. This hybrid supercapacitor also exhibits a good cycling performance and keeps 85% of initial capacity over 2000 cycles.     

TiO2 nanotubes promoting Pt/C catalysts for ethanol electro-oxidation in acidic media

In this paper, TiO₂ nanotubes/Pt/C (TNT/Pt/C) catalysts for ethanol electro-oxidation were prepared by co-mixing method in solution. TEM and XRD showed that uniform anatase TiO₂ nanotubes were about 100 nm in length and 8 nm in diameter and the TGA results indicated that the amount of H₂O contained in TiO₂ nanotubes was much more than that in anatase TiO₂. The composite catalysts activities were measured by cyclic voltammetry (CV), chronoamperometry and CO stripping voltammetry at 25 °C in acidic solutions. The results demonstrated that the TNT can greatly enhance the catalytic activity of Pt for ethanol oxidation and increase the utilization rate of platinum. The CO stripping test showed that the TNT can shift the CO oxidation potential to lower direction than TiO₂ does, which is helpful for ethanol oxidation.     

Electrochemical investigation of MnO2 electrode material for supercapacitors

MnO₂ electrode material is synthesized by low temperature solid state reaction between KMnO₄ and MnCl₂. Effects of the KMnO₄:MnCl₂ molar ratio on the structure, morphology and electrochemical properties of the as-prepared sample were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical tests. Results showed that the obtained MnO₂ is α-MnO₂, the average diameter is about 0.5-1.5 μm, which are constituted of nanoparticles of 20 nm. Under 100 mA g⁻¹, the specific capacitances of the prepared sample is 258.7, 219.6, 215.3, 198.5 and 209.5 F g⁻¹ at the KMnO₄/MnCl₂ molar ratio of 3:2, 2:1, 1:1, 1:2 and 2:3, respectively. And the MnO₂ sample with a KMnO₄/MnCl₂ molar ratio of 3:2 exhibits the best discharge capacitance and cycle performance. When the charge/discharge rate increases to 300 mA g⁻¹, the sample still remains initial discharge capacitance of 165.3 F g⁻¹, and the discharge capacitance is 145.9 F g⁻¹ after 200 cycles, the capacitance retention rate is 102.4% during the 20-200th cycles. Therefore, the MnO₂ sample is an excellent material for use in supercapacitors because of its large specific capacitance and good cycle performance.     

Hydrogenation behavior of high-energy ball milled amorphous Mg2Ni catalyzed by multi-walled carbon nanotubes

Amorphous Mg₂Ni alloy was successfully synthesized by means of mechanical alloying. Then, the multi-walled carbon nanotubes (MWCNTs) were added by high-energy ball milling to catalyze the amorphous alloy. The X-ray diffraction (XRD) spectroscopy reveal that the as-cast Mg₂Ni alloy has presented a completely amorphous state under specific conditions of high-energy ball milling process. Different process parameters of ball-to-powder ratio (10:1, 20:1, 40:1) and milling time have been attempted for the preparation of amorphous Mg₂Ni alloy. The results show that the milling time and ball-to-powder weight ratio have significantly influence on the amorphization process of crystalline Mg₂Ni alloy. Before and after the milling, phase compositions and microstructures of the prepared materials were characterized by XRD, scanning electron microscope (SEM), electron energy dispersion spectrum (EDS) and transition electron microscope (TEM) approaches. The morphology of composite Mg₂Ni/MWCNTs was investigated, the TEM images show that the MWCNTs imbed on the surface of the particles after milling for 1 h, and the MWCNTs with and without tubular structure have been observed. The hydrogen storage properties of amorphous Mg₂Ni alloys were improved by the catalytic effect of MWCNTs. The catalytic effect and mechanism of MWCNTs on the hydrogen storage properties of amorphous Mg₂Ni alloy are discussed and investigated.     

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.     

Impact of SO2 poisoning of platinum nanoparticles modified glassy carbon electrode on oxygen reduction

An extraordinary recovery characteristic of Pt-nanoparticles from SO₂ poisoning is introduced in this study. Platinum nanoparticles (nano-Pt) modified glassy carbon electrode (nano-Pt/GC) has been compared with polycrystalline platinum (poly-Pt) electrode towards SO₂ poisoning. Two procedures of recovery of the poisoned electrodes were achieved by cycling the potential in the narrow potential range (NPR, 0-0.8 V vs. Ag/AgCl/KCl (sat.)) and wide potential range (WPR, −0.2 to 1.3 V). The extent of recovery was marked using oxygen reduction reaction (ORR) as a probing reaction. SO₂ poisoning of the electrodes changed the mechanism of the oxygen reduction from the direct reduction to water to the stepwise reduction involving the formation of H₂O₂ as an intermediate, as indicated by the rotating ring-disk voltammetry. Using the WPR recovery procedure, it was found that two potential cycles were enough to recover 100% of the activity of the ORR on the nano-Pt/GC electrode. At the poly-Pt electrode, however, four potential cycles of the WPR caused only 79% in the current recovery, while the peak potential of the ORR was 130 mV negatively shifted as compared with the fresh poly-Pt electrode. Interestingly, the NPR procedure at the nano-Pt/GC electrode was even more efficient in the recovery than the WPR procedure at the poly-Pt electrode.     

High efficiency Pt-CeO2/carbon nanotubes hybrid composite as an anode electrocatalyst for direct methanol fuel cells

Pt-CeO₂/carbon nanotubes (Pt-CeO₂/CNTs), based on glucose polymerization in the inner pores of anodic aluminum oxide templates under hydrothermal conditions followed by carbonization at high temperature, were synthesized using as precursors H₂PtCl₆ reduced by NaBH₄ and CeCl₃ deposited by NaOH. Pt nanoparticles and CeO₂ units were inserted onto the outer surfaces and inner surfaces of the as-prepared carbon nanotubes (CNTs). The resulting structures were characterized by scanning electron microscopy (SEM). The electrocatalytic performances of the Pt-CeO₂/CNTs modified glass carbon electrodes were investigated for methanol oxidation by cyclic voltammetric and chronoamperometric measurements. It was found that compared with Pt/CNTs, the hybrid Pt-CeO₂/CNTs electrodes showed superior catalytic performance when the molar ratio of Pt to CeO₂ in the catalyst was about 2:1. The increased catalytic efficiency of Pt is likely to result from its combination with CeO₂.     

H2 adsorption mechanism in Mg modified multi-walled carbon nanotubes for hydrogen storage

Multi-walled carbon nanotubes (MWCNTs) with diameter of about 50 nm were synthesized using thermal chemical vapor deposition. We have investigated the influence of Mg doping to the MWCNTs on its hydrogen storage property. TEM micrographs showed that Mg was attached to the MWCNTs and discontinuous arrangement of the carbon walls was recognized in the MWCNTs. According to XPS and BET analyses, the surface functional groups and pore size of the Mg-MWCNTs are increased by interactions between the Mg and the MWCNT’s outer walls. The electrochemical discharging curves of the MWCNTs and Mg-doped MWCNTs revealed that the hydrogen storage capacity was 363 and 450 mAhg⁻¹, respectively. Volumetric technique determined that the hydrogen storage capacity of the MWCNTs and Mg-MWCNTs was 0.7 and 1.5 wt%, respectively. There are likely a couple of mechanism for Mg metal that used as dopant to pure MWCNTs, one involves increasing of adsorption binding energy and desorption temperature due to increasing defect sites (oxygen functional groups), while the second explains by electron transfer from metal atoms to carbon atoms resulting in a considerable increase in both the adsorption binding energy and desorption temperature.