Highly efficient CO2 bubble removal on carbon nanotube supported nanocatalysts for direct methanol fuel cell

In this paper, we investigate the CO₂ microbubble removal on carbon nanotube (CNT)-supported Pt catalysts in direct methanol fuel cells (DMFCs). The experiments involve the incorporation of near-catalyst-layer bubble visualization and simultaneous electrochemical measurements in a DMFC anodic half cell system, in which CH₃OH electro-oxidation generate carbon dioxide (CO₂) microbubbles. We observe rapid removal of smaller CO₂ bubble sizes and less bubble accumulation on a Pt-coated CNT/CC (Pt/CNT/CC, CC means carbon cloth) electrode. The improved half cell performances of the high CO₂ microbubble removal efficiency on the CNT-modified electrode (Pt/CNT/CC) were 34% and 32% higher than on Pt/CC and Pt/CP electrodes, respectively.     

The performance of Pt nanoparticles supported on Sb2O5.SnO2, on carbon and on physical mixtures of Sb2O5.SnO2 and carbon for ethanol electro-oxidation

Pt nanoparticles were supported on Sb₂O₅.SnO₂ (ATO), on carbon and on physical mixtures of ATO and carbon by an alcohol-reduction process using ethylene glycol as reducing agent. The obtained materials were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Their performance for ethanol oxidation was investigated at room temperature by chronoamperometry and in a direct ethanol fuel cell (DEFC) at 100 °C. Pt nanoparticles supported on a physical mixture of ATO and carbon showed a significant increase of performance for ethanol oxidation compared to Pt nanoparticles supported on ATO or on carbon.     

Hydrogenation and dehydrogenation of Mg2Co nanoparticles and carbon nanotube composites

This study investigated the hydrogenation and dehydrogenation behavior of Mg₂Co nanoparticles and carbon nanotube (CNT) composites using temperature-programmed deposition, Raman spectroscopy, and X-ray diffraction (XRD). We used the mechanical alloying method to prepare nanosized Mg₂Co particles on CNTs with three loadings of alloys. The introduction of CNTs showed dehydrogenation, hydrogen desorption starting at 370 °C, with the majority of hydrogen being below 500 °C. This can be explained by the fact that Mg₂Co alloy deposited on CNT surface induced the dissociation of hydrogen into two atoms, which were spilt over and then intercalated into the interlayer of CNT. Accordingly, the atomic intercalation enabled the reduction of the hydrogen desorption activation barrier. The spillover mechanism of hydrogen storage can be confirmed by XRD and Raman spectroscopy because of larger interspacing (d_0 0 2) and weaker graphite degree (I_D/I_G) of CNTs after hydrogenation.     

Hydrogenation of carbon monoxide over cobalt nanoparticles supported on carbon nanotubes

This paper describes a catalytic reaction of hydrogen and carbon monoxide (Fischer-Tropsch synthesis (FTS)) over carbon nanotubes (CNTs) supported cobalt nanoparticles. We have investigated the effect of calcination of the catalysts on FTS performance using X-ray diffraction (XRD), H₂ chemisorption, temperature programmed reduction (TPR), temperature programmed oxidation (TPO), and transmission electron microscopy (TEM) techniques. With the increase of outer diameter of CNTs, specific surface area of the catalyst decreases while Co particle size increased accompanying with a decrease in CO conversion. The FTS performance is similar for samples calcined in N₂ or air at temperature below 550 °C. Over 550 °C, the results are much different in that the Co/CNTs can keep its activity due to the unchanged CNTs structure in N₂ while the Co/CNTs almost lose activity owing to the loss of CNTs structure and sintering of cobalt oxide clusters in air.     

Hydrothermal synthesis of SnO2–V2O5 mixed oxide and electrochemical screening of carbon nano-tubes (CNT), V2O5, V2O5–CNT,and SnO2–V2O5–CNT electrodes for supercapacitor applications

Nano-scaled SnO₂–V₂O₅ mixed oxide is synthesized by a hydrothermal method in an autoclave. For comparative evaluation, V₂O₅ single oxide is prepared by a conventional process from ammonium vanadate. The capacitive behaviour of the following electrodes is studied by cyclic voltammetry in 0.1 M KCl solutions: carbon nano-tubes (CNT), V₂O₅, V₂O₅–CNT, and SnO₂–V₂O₅–CNT. At a scan rate of 100 mV s⁻¹, the SnO₂–V₂O₅–CNT electrode provides the best performance, viz., 121.4 F g⁻¹. The nano-scaled mixed oxide is characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Raman spectra.     

Hydrogenation of LiH/Al catalyzed with TiN,TiMn2 and LaNi5

In order to investigate the catalytic effect of TiN, TiMn₂ and LaNi₅ on the hydrogen storage capacity of LiAlH₄, 2 mol% of the catalyst was milled with LiH/Al and then hydrogenated in Me₂O. Doping with TiN, TiMn₂ or LaNi₅ led to substantial hydrogenation of LiH/Al in accordance with the formation of LiAlH₄. In each case the amount of hydrogen absorbed was dependent on the catalyst and the ball-to-powder ratio used during milling. A high ball-to-powder ratio results in an improvement in the hydrogen storage capacity of LiAlH₄. For each ball-to-powder ratio the highest hydrogen storage capacity was recorded for the TiN-catalyzed sample; hydrogen storage capacity increased from 3.2 to 4.8 to 6.0 wt.% H as the ball to-powder ratio increased from 10:1 to 20:1 to 40:1. The high levels of hydrogenation of LiH/Al catalyzed with TiN, TiMn₂ and LaNi₅ are remarkable because for the LiAlH₄ system only a TiCl₃ catalyst has previously been shown to result in rehydrogenation of the dehydrogenated products to LiAlH₄.     

Effect of Nb2O5 on MgH2 properties during mechanical milling

Recently, it was shown that hydrogen absorption–desorption kinetics in magnesium were improved by milling magnesium hydride (MgH₂) with transition metal oxides. Herein, we investigate the role of the most effective of these oxides, Nb₂O₅ when added in larger volume fraction. The effect of Nb₂O₅ on magnesium crystalline structure, particle size and (ab)desorption properties has been characterised. Moreover, we report that pure MgH₂ can also show fast hydrogen sorption kinetics after a long milling time. The effects of Nb₂O₅ on MgH₂ sorption properties are rationalised in a new approach considering Nb₂O₅ as a dispersing agent, which helps reduce MgH₂ particle size during milling.     

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

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

An in depth investigation of deactivation through carbon formation during the biogas dry reforming reaction for Ni supported on modified with CeO2 and La2O3 zirconia catalysts

The dry reforming of biogas on a Ni catalyst supported on three commercially available materials (ZrO₂, La₂O₃ZrO₂ and CeO₂ZrO₂), has been investigated, paying particular attention to carbon deposition. The DRM efficiency of the catalysts was studied in the temperature range of 500–800 °C at three distinct space velocities, and their time-on-stream stability at four temperatures (550, 650, 750 and 800 °C) was determined for 10 or 50 h operation. The morphological, textural and other physicochemical characteristics of fresh and spent catalysts together with the amount and type of carbon deposited were examined by a number of techniques including BET-BJH method, CO₂ and NH₃-TPD, XPS, SEM, TEM, STEM-HAADF, Raman spectroscopy, and TGA/DTG. The impact of the La₂O₃ and CeO₂ modifiers on the DRM performance and time-on-stream stability of the Ni/ZrO₂ catalyst was found to be very beneficial: up to 20 and 30% enhancement in CH₄ and CO₂ conversions respectively, accompanied with a CO-enriched syngas product, while the 50 h time-on-stream catalytic performance deterioration of ～30–35% on Ni/ZrO₂ was limited to less than ～15–20% on the La₂O₃ and CeO₂ modified samples. Their influence on the amount and type of carbon formed was substantial: it was revealed that faster oxidation of the deposited carbon at elevated temperatures occurs on the modified catalysts. Correlations between the La₂O₃ and CeO₂-induced modifications on the surface characteristics and physicochemical properties of the catalyst with their concomitant support-mediated effects on the overall DRM performance and carbon deposition were revealed.     

H2 production from methane pyrolysis over commercial carbon catalysts: Kinetic and deactivation study

Hydrogen production from catalytic methane decomposition (DeCH₄) is a simple process to produce high purity hydrogen with no formation of carbon oxides (CO or CO₂). However, to completely avoid those emissions, the catalyst must not be regenerated. Therefore, it is necessary to use inexpensive catalysts, which show low deactivation during the process. Use of carbon materials as catalysts fulfils these requirements.     

Visible light-induced hydrogen over CuFeO2 via S2O3 oxidation

The properties of CuFeO₂ have been studied according to the catalytic hydrogen production upon visible light. CuFeO₂ with a low band gap E_g, a good chemical stability and a suitable flat band potential appears as a suitable candidate. The potential of photoelectrons allows favorably a thermodynamically H₂-evolution from alkaline thiosulfate S₂O₃²⁻ solution. There is a major difference between pure and loaded oxide with some metal catalysts. Our best results have been obtained with unloaded CuFeO₂ at 50 °C and pH 13.60. Thiosulfate S₂O₃²⁻ ions can be oxidized to sulfite SO₃²⁻ and subsequently to sulfate SO₄²⁻ and the electronic exchange occurs via mediation of surface states. The quite high H₂-formation at the beginning shows a tendency towards saturation, it competes with SO₃²⁻ produced by parallel oxidation of S₂O₃²⁻.     

European CO2 prices and carbon capture investments

We assess the option to install a carbon capture and storage (CCS) unit in a coal-fired power plant operating in a carbon-constrained environment. We consider two sources of risk, namely the price of emission allowance and the price of the electricity output. First we analyse the performance of the EU market for CO₂ emission allowances. Specifically, we focus on the contracts maturing in the Kyoto Protocol's first commitment period (2008 to 2012) and calibrate the underlying parameters of the allowance price process. Then we refer to the Spanish wholesale electricity market and calibrate the parameters of the electricity price process.We use a two-dimensional binomial lattice to derive the optimal investment rule. In particular, we obtain the trigger allowance prices above which it is optimal to install the capture unit immediately. We further analyse the effect of changes in several variables on these critical prices, among them allowance price volatility and a hypothetical government subsidy.We conclude that, at current permit prices, immediate installation does not seem justified from a financial point of view. This need not be the case, though, if carbon market parameters change dramatically, carbon capture technology undergoes significant improvements, and/or a specific governmental policy to promote these units is adopted.     

H2 storage on single- and multi-walled carbon nanotubes

The adsorption of hydrogen on single-walled and multi-walled carbon nanotubes (CNTs) was investigated at 77 and 298 K, in the pressure range of 0–1000 Torr. The adsorption isotherms indicate that adsorption follows the Langmuir model. Hydrogen uptakes were found to depend strongly on the nature of the CNTs. Single-walled CNTs adsorb significantly higher quantities of hydrogen per unit mass of the solid, while the opposite is true on a per unit surface area basis. This observation implies that adsorption takes place selectively on specific sites on the surface. The hydrogen uptake capacity of CNTs was also found to be affected by the purity of the materials, increasing with increasing purity. Temperature programmed desorption indicated that relatively strong adsorption bonds develop between adsorbent and adsorbate and that a single type of adsorption site exists on the solid surface.     

Selective CO oxidation on Cu promoted Pt/Al2O3 and Pt/Nb2O5 catalysts

Pt–Cu catalysts supported on Al₂O₃ and Nb₂O₅ were studied for use in selective CO oxidation. The addition of copper enhanced the activity and selectivity of Pt–Cu/Nb₂O₅ at lower temperatures when compared to Pt/Nb₂O₅. On the other hand, copper addition was not beneficial in the case of Al₂O₃ supported catalysts.     

Studies on the de/re-hydrogenation characteristic of Mg(NH2)2/LiH mixture admixed with carbon nanofibres

The effect of carbon nanofibres (CNFs) on the de/re-hydrogenation characteristics of 1:2 magnesium amide (Mg(NH₂)₂) and lithium hydride (LiH) mixture is investigated. It is found that the desorption as well as absorption characteristic of the 1:2 Mg(NH₂)₂/LiH mixture is improved with admixing of different shaped (planar and helical) CNFs separately. The different shaped CNFs were synthesized through catalytic decomposition of acetylene gas over LaNi₅ alloy. The synthesized CNFs contain Ni-metal nano particles. Among two different types of nanofibres namely planar carbon nanofibres (PCNFs) and helical carbon nanofibres (HCNFs), the later was found to act as a better catalyst. The decomposition temperature of the pristine Mg(NH₂)₂/LiH mixture is ∼250 °C, reduced to 150 and 140 °C for the PCNF and HCNF admixed Mg(NH₂)₂/LiH mixture respectively. The activation energy for dehydrogenation reaction was found to ∼97.2 kJ/mol, which is further reduced to ∼67 and ∼65 kJ/mol for the PCNF and HCNF admixed Mg(NH₂)₂/LiH mixture respectively. The lowering of decomposition temperature and enhancement in desorption kinetics, with admixing of different shaped CNFs are described and discussed.     

Carbon nanotube formation during propane decomposition on boron-modified Co/Al2O3 catalysts: A kinetic study

In recent years, catalytic decomposition of light hydrocarbons has been explored as an interesting alternative to produce CO_x-free hydrogen along with valuable carbonaceous nanomaterials. In this contribution, we report on the effect of B on the catalytic performance of Co/Al₂O₃ catalysts during propane decomposition. Unpromoted and B-promoted (0.5–5 wt.% B) Co catalysts were prepared by incipient wetness impregnation and thoroughly characterized using XRD, XPS, TPR, H₂ chemisorption, CO-FTIR, Raman, TPO and TEM. The kinetic data were analysed using a phenomenological kinetic model. The kinetic results indicate the existence of an optimal amount of boron (ca. 2%) that maximizes both activity and stability of the catalyst. Based on the analysis of the kinetic parameters, it can be concluded that the presence of boron slows down the carburization and carbon diffusion steps, decreasing the amount of CNTs formed. However, the addition of Boron simultaneously decreases the formation of encapsulating coke, which is the cause of the catalyst deactivation.     

Effect of carbon nanotube on the electrochemical performance of C-LiFePO4/graphite battery

This paper describes the fabrication and testing of C-LiFePO₄/graphite battery with different conductive carbon additives: carbon nanotube (CNT) or carbon black (CB). The discharge capacity, rate capability and cyclic performance of the battery were investigated. Compared with the batteries with CB additive, those with CNT additive show better electrochemical performances with capacity retention ratio of 99.2% after 50 cycles, and the ratio of discharge capacity at 0.1 C rate to that at 1 C rate is 94.6%. The reason for the difference in electrochemical property was studied with cyclic voltammagrams and AC impedance. It was found that, with CNT additive, the polarization voltage was decreased from 0.3 to 0.2 V, and the impedance was decreased from 423.2 to 36.88 Ω. The structures of active materials after cycling were characterized using XRD. The better crystal retaining of LiFePO₄ was found in the active materials with CNT added.     

Bonding in PdH2 and Pd2H2 systems adsorbed on carbon nanotubes: Implications for hydrogen storage

This work presents a bonding study of hydrogen adsorption processes on palladium decorated carbon nanotubes by using the density functional theory (DFT). First, we considered simple decoration models involving single palladium atoms or palladium dimers, and then we analyzed the adsorption of several molecular and dissociated hydrogen coordination structures, including Kubas-type complexes. In all cases we computed the energy, bonding and electronic structure for the different nanotube-supported hydrogen–palladium systems. Our results show that Pd(H₂) and Pd₂(H₂) complexes with relaxed but not dissociated H–H bonds are the most stable adsorbed systems. The role of s, p and d orbitals on the bonding mechanism for all adsorbates and substrates was also addressed. We found intermolecular donor–acceptor C–Pd and Pd–H delocalizations after adsorption. We also studied the palladium clustering effect on the hydrogen uptake based on Kubas-type bonding.     

Study on carbon nanotube reinforced phenol formaldehyde resin/graphite composite for bipolar plate

Using carbon nanotubes (CNTs) after different Fenton treatments as a reinforcement and a phenol formaldehyde resin/graphite (PF/G) composite as matrix, a new composite for bipolar plate was formed by hot-pressing. The effects of Fenton, Fenton/ultrasonic and Fenton/ultraviolet treatments on the surface of the CNTs, and the bend strength and conductivity of bipolar plate composite produced using them were investigated. It was found that Fenton/UV treatment was an effective and advanced oxidation process, which could generate a large quantity of hydroxyl groups and few carboxyl groups on the sidewalls of the CNTs, but without severe damage. The functional groups on CNTs after Fenton/ultraviolet treatment can improve the interfacial adhesion between CNTs and matrix, which can improve the bend strength, but does not play an important role in the improvement of the conductivity. The bend strength and conductivity of the composite with 3% CNTs after Fenton/ultraviolet treatment are 68.6 MPa and 145.2 s cm⁻¹, respectively, when pressed at 240 °C for 60 min.     

Influence of multiple oxide (Cr2O3/Nb2O5) addition on the sorption kinetics of MgH2

The influence of multiple additions of two oxides, Cr₂O₃ and Nb₂O₅, as additives on the hydrogen sorption kinetics of MgH₂ after milling was investigated. We found that the desorption kinetics of MgH₂ were improved more by multiple oxide addition than by single addition. Even for the milled MgH₂ micrometric size powders, the high hydrogen capacity with fast kinetics were achieved for the powders after addition of 0.2 mol% Cr₂O₃ + 1 mol% Nb₂O₅. For this composition, the hydride desorbed about 5 wt.% hydrogen within 20 min and absorbed about 6 wt.% in 5 min at 300 °C. Furthermore, the desorption temperature was decreased by 100 °C, compared to MgH₂ without any oxide addition, and the activation energy for the hydrogen desorption was estimated to be about 185 kJ mol⁻¹, while that for MgH₂ without oxide was about 206 kJ mol⁻¹.