Mn3O4 nanosheets coated on carbon nanotubes as efficient electrocatalysts for oxygen reduction reaction

MnO-MnC_x coated carbon nanotubes (MnO/MnC_x/CNTs) nanocomposites were prepared by a one-pot deposition method. The coating consisted of MnO, Mn₅C₂, Mn₁₅C₄ and Mn₂₃C₆ was formed on the surface of CNTs by heating a mixture of Mn particles and CNTs at 600 °C for 40 min under vacuum. Then after heated MnO/MnC_x/CNTs in air at 350 °C for 2 h, MnO nanoparticles were partially converted to Mn₃O₄ nanosheets. Then Mn₃O₄-MnC_x coated carbon nanotubes (Mn₃O₄/MnC_x/CNTs) composed of interconnected nanosheets structure were successfully synthesized by a two-step method of one-pot deposition and heat post-treatment. The Mn₃O₄/MnC_x/CNTs showed better oxygen reduction reaction performance in alkaline condition than MnO/MnC_x/CNTs and pristine CNTs. Besides, the formed MnC_x (Mn₅C₂ and Mn₂₃C₆) by one-pot deposition method provided a strong interface bonding between Mn₃O₄ and CNTs, leading to improved stability of Mn₃O₄/MnC_x/CNTs as an electrode material.     

In situ synthesis of CoFe2O4 nanoparticles embedded in N-doped carbon nanotubes for efficient electrocatalytic oxygen reduction reaction

The development of electrocatalyst possessing superior oxygen reduction reaction (ORR) activity is highly desirable due to the low sluggish kinetics at the cathode of fuel cell. Here, CoFe₂O₄ nanoparticles embedded in N-doped carbon nanotubes electrocatalyst (denoted as CoFe₂O₄-NC) is synthesized via polymerization of pyrrole, absorbing metal ion and annealing under Ar/NH₃ atmosphere. By in situ integrating the catalytically active CoFe₂O₄ nanoparticles with the N-doped carbon nanotubes and enhancing electrical conductivity, the as-obtained electrocatalyst exhibits excellent ORR activity and long-term stability with a half-wave potential of 0.86 V and 10 h continuous cycling, outperforming the reported similar catalysts. This work opens a new path for the expansion of low cost and efficient ORR electrocatalysts to substitute Pt-based metals for energy storage and conversion devices.     

Single walled carbon nanotubes as supports for metal hydride electrodes

The electrochemical generation and the storage of hydrogen employing metal hydrides has become a good alternative attending the requirement to search for new sources of clean energy. This work is devoted to study the hydrogen storage as hydrides in an AB₅-type metal alloy (MmNi_4.1Co_0.4Mn_0.4Al_0.5). The behaviour of the alloy containing electrode was evaluated employing several electrodes containing the alloy and diverse carbons. Carbons were prepared by using Single Walled Carbon Nanotubes (SWCNT) with different PTFE percentages (15%, 25% y 33%) and Carbon Vulcan XC72® with 33% of PTFE (VT). Several electrochemical techniques as Cyclic Voltammetry (CV), Charge–Discharge cycles and Electrochemical Impedance Spectroscopy (EIS) were used. Results demonstrate that at low discharge currents, electrodes containing SWCNT exhibit better hydrogen storage than Vulcan XC72® containing electrodes. Studies made with carbon supports only show that this little but not disregarded differences are related to different hydrogen sorption behaviour of SWCNT and Vulcan XC72®. From the kinetic point of view, Vulcan XC72® containing electrodes have a better behaviour than those prepared with SWCNT. On the other hand, the optimal percentage of PTFE for SWCNT was determined to be 25%.     

Investigation on platinum loaded multi-walled carbon nanotubes for hydrogen storage applications

Molecular hydrogen uptake of modified carbon nanotubes is a prospect for efficient hydrogen storage in fuel cell vehicles. In this study, a simple and efficient method to decorate the surface of multi-walled carbon nanotubes (MWNT) with platinum nanoparticles is presented. To load the Pt nanoparticles, hexachloroplatinic acid (H₂PtCl₆·6H₂O) is used as a precursor. Surface morphology of these Pt loaded MWNT is observed using Scanning and Transmission Electron Microscopy. Both samples are also characterized by X-Ray Diffraction. Thermal Gravimetric Analysis results indicate that both as purchased MWNT and Pt loaded MWNT have decomposition temperature higher than 500 °C in air. N₂ adsorption experiments yields a BET area of the sample close to 500 m²/g. This MWNT/Pt sample was reduced in 10% of H₂ in Ar, flowing at 900 °C in a tubular furnace for 1 h before hydrogen adsorption measurements. Hydrogen uptake of MWNT/Pt was measured at 2.5 MPa and 77 K. This hydrogen uptake isotherm is also compared with measurements at ambient temperature.     

Influence of nitrogen doping on carbon nanotubes towards the structure,composition and oxygen reduction reaction

Nitrogen-doped carbon nanotubes (NCNTs) as noble-metal free catalysts were synthesised via chemical vapour deposition using iron (II) phthalocyanine as a metal catalyst for growth of the nanotubes. The synthesis process was performed in one step in a tube furnace using different chemical precursors (aniline, diethylamine and ethylenediamine) as nitrogen sources. The NCNT samples were physically characterised using scanning electron microscopy, transmission electronic microscopy and X-ray photoelectron spectroscopy. Cyclic voltammetry measurements were conducted to investigate the oxygen reduction reaction (ORR), and they showed that the electrocatalytic activities depend on the structural and morphological changes in the NCNTs. The results showed that NCNTs synthesised from ethylenediamine precursors exhibit an ORR scheme proceed via indirect four electron transfer process in acidic media, which implies that these NCNTs are a candidate for serving as the cathodic catalyst in PEMFCs. The highly active NCNTs possess unique characteristics, including a high concentration of surface defects with high pyridinic-N and pyridinic-N-oxides configurations that serve as active sites for ORR activity in acidic media.     

Manganese dioxide-coated carbon nanotubes as an improved cathodic catalyst for oxygen reduction in a microbial fuel cell

To develop an efficient and cost-effective cathodic electrocatalyst for microbial fuel cells (MFCs), carbon nanotubes (CNTs) coated with manganese dioxide using an in situ hydrothermal method (in situ MnO₂/CNTs) have been investigated for electrochemical oxygen reduction reaction (ORR). Examination by transmission electron microscopy shows that MnO₂ is sufficiently and uniformly dispersed over the surfaces of the CNTs. Using linear sweep voltammetry, we determine that the in situ MnO₂/CNTs are a better catalyst for the ORR than CNTs that are simply mechanically mixed with MnO₂ powder, suggesting that the surface coating of MnO₂ onto CNTs enhances their catalytic activity. Additionally, a maximum power density of 210 mW m⁻² produced from the MFC with in situ MnO₂/CNTs cathode is 2.3 times of that produced from the MFC using mechanically mixed MnO₂/CNTs (93 mW m⁻²), and comparable to that of the MFC with a conventional Pt/C cathode (229 mW m⁻²). Electrochemical impedance spectroscopy analysis indicates that the uniform surface dispersion of MnO₂ on the CNTs enhanced electron transfer of the ORR, resulting in higher MFC power output. The results of this study demonstrate that CNTs are an ideal catalyst support for MnO₂ and that in situ MnO₂/CNTs offer a good alternative to Pt/C for practical MFC 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.     

Nitrogen-doped multi-walled carbon nanocoils as catalyst support for oxygen reduction reaction in proton exchange membrane fuel cell

Multi-walled carbon nanocoils (MWNCs) are synthesized by chemical vapour deposition and nitrogen-doped MWNCs (N-MWNCs) are obtained by nitrogen plasma treatment. MWNCs and N-MWNCs are used as catalyst supports for platinum nanoparticles. Pt nanoparticles are dispersed over these support materials using the conventional chemical reduction technique and then used for the oxygen reduction reaction in proton-exchange membrane fuel cells. The morphology and structure of the MWNC-based powder samples are studied by means of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Full cells are constructed with Pt-loaded MWNC/N-MWNC and the results are discussed. A maximum power density of 550 and 490 mW cm⁻² is obtained with Pt/N-MWNC and Pt/MWNC as the ORR catalyst, respectively. The improved performance of a fuel cell with a N-MWNC catalyst support can be attributed to the creation of pyrrolic nitrogen defects due to the nitrogen plasma treatment. These defects act as good anchoring sites for the deposition of Pt nanoparticles and to the increased electrical conductivity and improved carbon-catalyst binding.     

High-performance N,P-CNL nanocomposites as catalyst for oxygen reduction reaction in fuel cell

Transition metal and heteroatom codoped carbon materials have become the most promising materials to replace commercial platinum carbon (Pt / C) catalysts due to their low cost, high stability, and methanol resistance. In this work, iron-nitrogen and phosphorus codoped carbon nanorod-layer composites (N, P-CNL) derived from phosphorus-doped polyaniline (P-PANI) by phytic acid (PA) and iron salt were successfully obtained after high-temperature pyrolysis. As a result, the N, P-CNL materials exhibited good electrocatalytic performance due to abundant active sites. The N, P-CNL with 50% mass filling ratio of iron salt (named as N, P-CNL-1:1) displayed an enhanced limiting current density of −5.97 mA cm⁻² at 1600 rpm and outstanding onset potential (−0.004 V) and oxygen reduction peak potential (−0.144 V). In general, this work can give insights into understanding the mechanism of codoped catalysts and synthesis the catalyst with excellent long-term stability and resistance to methanol crossover and poisoning better than commercial Pt/C.     

Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells

Nitrogen doped mesoporous carbons are employed as supports for efficient electrocatalysts for oxygen reduction reaction. Heteroatom doped carbons favour the adsorption and reduction of molecular oxygen on Pt sites. In the present work, nitrogen doped mesoporous carbons (NMCs) with variable nitrogen content were synthesized via colloidal silica assisted sol-gel process with Ludox-AS40 (40 wt% SiO₂) as hard template using melamine and phenol as nitrogen and carbon precursors, respectively. The NMC were used as supports to prepare Pt/NMC electrocatalysts. The physicochemical properties of these materials were studied by SEM, TEM, XRD, BET, TGA, Raman, XPS and FTIR. The surface areas of 11 wt% (NMC-1) and 6 wt% (NMC-2) nitrogen doped mesoporous carbons are 609 m² g^?1 and 736 m² g^?1, respectively. The estimated electrochemical surface areas for Pt/NMC-1 and Pt/NMC-2 are 73 m² g^?1 and 59 m² g^?1, respectively. It is found that Pt/NMC-1 has higher ORR activity with higher limiting current and 44 mV positive onset potential shift compared to Pt/NMC-2. Further, the fuel cell assembled with Pt/NMC-1 as cathode catalyst delivered 1.8 times higher power density than Pt/NMC-2. It is proposed that higher nitrogen content and large pyridinic nitrogen sites present in NMC-1 support are responsible for higher ORR activity of Pt/NMC-1 and high power density of the fuel cell using Pt/NMC-1 cathode electrocatalyst. The carbon support material with high pyridinic content promotes the Pt dispersion with particle size less than 2 nm.     

High performance catalysts based on Fe/N co-doped carbide-derived carbon and carbon nanotube composites for oxygen reduction reaction in acid media

The key issue of modern electrochemical technology is clean energy production and storage. Proton exchange membrane fuel cells (PEMFC) offer a way to produce electricity from hydrogen, but are hindered by the sluggish reduction of oxygen into water on the cathode, which requires Pt/C catalysts. Iron-nitrogen-carbon (Fe-N-C) catalysts have been shown in recent years to be viable alternatives. Here, we present highly performing Fe-N-C catalysts based on composite materials synthesised from carbide-derived carbon (CDC) and carbon nanotubes (CNT). B₄C, Mo₂C and TiC, which yield CDC materials with different porosity were chosen as the starting carbides, which are then doped with Fe, N and composited with CNTs using ball-milling and pyrolysis. 1,10-phenanthroline (Phen) and dicyandiamide (DCDA) serve as the nitrogen sources and Fe(II)acetate as the iron source. The catalyst derived from TiC shows a remarkable half-wave potential for oxygen reduction of 0.8 V vs RHE, which shifts negative 36 mV during 5000 potential cycles at 70 °C, while the composite material derived from it is more stable with a shift of only 15 mV during the same period.     

Co/multi-walled carbon nanotubes as highly efficient catalytic nanoreactor for hydrogen production from formic acid

Formic acid is well-recognized as safe and convenient hydrogen carrier. Development of active and cost-effective catalysts for formic acid to hydrogen conversion is important problem of hydrogen energy field. Herein, we report on new Co catalysts supported on oxidized multi-walled carbon nanotubes (MWCNTs), which demonstrate high efficiency in the gas-phase formic acid decomposition affording molecular hydrogen. Various parameters of the catalysts, Co loading, MWCNTs structure, and nanotubes treatment conditions, have been investigated in terms of their influence on the catalytic properties. The catalysts morphology has been characterized with a set of physicochemical methods. It is found that the catalytic activity of Co particles depends on their electronic state and location on the support. Co species located inside the MWCNTs channels are less active than Co species stabilized on the outer surface. An increase in the content of Co nanoparticles on the MWCNT outer surface leads to a higher catalytic activity.     

Hydrogen storage in microwave-treated multi-walled carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs) treated by microwave and heat treatment were used for hydrogen storage. Their storage capacity was measured using a quadruple quartz crystal microbalance in a moisture-free chamber at room temperature and at relatively low pressure (0.5 MPa). Deuterium was also used to monitor the presence of moisture. The hydrogen storage capacity of the microwave-treated MWCNTs was increased to nearly 0.35 wt% over 0.1 wt% for the pristine sample and increased further to 0.4 wt%, with improved stability after subsequent heat-treatment. The increase in the storage capacity by the microwave treatment was mostly attributed to the introduction of micropore surfaces, while the stability improvement after the subsequent heat treatment was related to the removal of functional groups. We also propose a measurement method that eliminates the moisture effect by measuring the storage capacity with hydrogen and deuterium gas.     

Ultrasonic synthesis of nitrogen-doped carbon nanofibers as platinum catalyst support for oxygen reduction

Oxygen- and nitrogen-containing groups are successfully introduced onto the carbon nanofiber (CNF) surfaces by sonochemical treatment in mixed acids (concentrated sulfuric acid and nitric acid) and ammonia, respectively. Pt electrocatalysts supported on the acid-treated CNF (CNF-O) and ammonia-treated CNF (CNF-ON) are prepared and the effect of CNF surface functional groups on the electrocatalytic activities of supported catalysts for oxygen reduction reaction (ORR) is investigated. High resolution transmission electron microscopy reveals that Pt particles are uniformly dispersed on the two CNF supports and the CNF-ON supported Pt nanoparticles have a smaller average particle size and a more uniform particle size distribution. Cyclic voltammetric analysis shows the Pt/CNF-ON has a larger electrochemically active surface area than Pt/CNF-O. Rotating disk electrode measurements show that the Pt/CNF-ON exhibits a considerably higher electrocatalytic activity toward ORR as compared with Pt/CNF-O. It is believed that the good electrocatalytic activity of Pt/CNF-ON can be attributed to the smaller Pt particle size and more uniform particle size distribution, to the synergistic effect and the enhanced Pt-CNF-ON interaction, and to the unique structural and electronic properties of CNF-ON.     

Facile synthesis of Pd supported on Shewanella as an efficient catalyst for oxygen reduction reaction

While noble metals loaded on carbon-based supports are commonly used as oxygen reduction catalysts for fuel cell cathodes, the preparation process is complicated and expensive cost. In this paper, Pd²⁺ was first adsorbed on Shewanella by its adsorption characteristics, then the Pd supported on Shewanella catalyst was obtained after carbonization at 600 °C and hydrogen reduction at 200 °C. The Shewanella cells retain the rod shape of bacteria following pyrolysis under high temperatures, while N and Pd heteroatoms are uniform distribution on the carbon matrix. As a result, Pd supported on Shewanella catalyst exhibits excellent electrocatalytic activity for ORR via a dominant four-electron oxygen reduction pathway in alkaline medium. More importantly, the mass activity of the prepared catalyst was 5.8 times higher than that of commercial Pd/C, and its stability was also better than the Pd/C, which could be promising alternatives to costly Pt-based electrocatalysts for ORR.     

Three-dimensional nitrogen-doped carbon nanotubes/carbon nanofragments complexes for efficient metal-free electrocatalyst towards oxygen reduction reaction

Heteroatom-doped carbon materials as one of the most promising oxygen reduction reaction (ORR) catalysts have attracted much attention. Rational design and exploration of suitable heteroatom-doped carbon materials greatly affects their ORR performance. Herein, we successfully prepared nitrogen-doped carbon nanotubes/carbon nanofragments (NCNT/CNF) complexes by a pyrolysis process using oxidized open-ended carbon nanotubes (OCNT)/oxidized carbon nanofragments (OCNF) hybrids as carbon precursors. The effect of carbon precursors on the synthesis of the corresponding nitrogen-doped carbon products was systematically investigated. The result showed the OCNT retained good conductivity, while the OCNF offered adequate structure defects for efficient post-doping. Benefiting from the co-merits of sole constitute, the obtained NCNT/CNF_1-15 (1–15 refers to the mass ratio) complexes possessed a typical three-dimensional architecture and much increased specific surface area, which facilitated reactant/electrolyte infiltration and ion/electron transfer. More importantly, they built the most optimized balance on ORR catalytic sites and conductivity. Thus, the NCNT/CNF_1-15 complexes showed much enhanced ORR performance. Clearly, our work provides a good guidance on the design of advanced heteroatom-doped carbon-based ORR catalysts.     

Tin-nitrogen/carbon for superior oxygen reduction reaction at fuel cell cathode

This work reports on the synthesis of tin-nitrogen/carbon (Sn–N/C) catalysts suitable for the electroreduction of molecular oxygen at the cathode of proton exchange membrane fuel cells. The catalysts were synthesized through a simple pyrolysis process of folic acid as the carbon and nitrogen source, tin chloride as a tin source and Vulcan carbon as the substrate. The synthesized catalyst exhibited excellent oxygen reduction activity with a half wave potential of 0.82 V and a mass activity of 15.5 mA mg⁻¹. Successful application at the cathode of a self-breathing fuel cell further confirmed the superior performance of this catalyst leading to a power density of 29.4 mW cm⁻². This is very comparable to the reference platinum/Vulcan carbon catalyst (28.4 mW cm⁻²). In addition, this Sn–N/C catalyst showed good stability under accelerated stress tests with only a 12% decrease in fuel cell performance after 10,000 cycles. The superior performance was assumed to be due to the presence of both metal-nitrogen and nitrogen-carbon active sites, which facilitate the four-electron path of the oxygen reduction reaction.     

Development of tellurium-modified carbon catalysts for oxygen reduction reaction in PEM fuel cells

Tellurium (Te)-modified carbon catalyst for oxygen reduction reaction was prepared through chemical reduction of telluric acid followed by the pyrolysis process at elevated temperatures. The catalyst was found to be active for oxygen reduction reaction. High-temperature pyrolysis plays a crucial role in the formation of the active sites of the catalysts. When the pyrolysis was conducted at 1000 °C, the catalyst exhibited the onset potential for oxygen reduction as high as 0.78 V vs. NHE and generated less than 1% H₂O₂ during oxygen reduction. The performance of the membrane–electrode assembly prepared with the Te-modified carbon catalyst was also evaluated.