Carbon Nanotube Catalyst Research Articles

Mechanistic insights into the electrolyte effects on the electrochemical nitrogen reduction reaction using copper hexacyanoferrate/f-MWCNT nano-composites.

Developing an efficient, selective, and stable electrocatalysis system for the electrocatalytic N2 reduction reaction (ENRR) is a promising strategy for the green and sustainable production of ammonia. The activity, selectivity, and stability of various electrocatalysts in different electrolyte solvents, mainly acidic and alkaline electrolytes, are commonly compared in the literature. However, a mechanistic insight into the effect of these electrolytes on ENRR activity is lacking. Herein we demonstrate that the acidity or alkalinity of the electrolyte is a key factor in determining the rate-limiting step and, by extension, the ENRR performance of an electrochemical setup for the electroproduction of ammonia. Our results from ex situ X-ray photoelectron, Raman, and FTIR spectroscopy analysis of the fresh and spent Cu-hexacyanoferrate Prussian blue analogue-decorated functionalized carbon nanotube (CuFe PBA/f-CNT) catalyst reveal that NH4+-species are more strongly adsorbed on the catalyst surface during the ENRR in acidic than in alkaline electrolytes. The results of our detailed rotating ring-disc electrode voltammetry studies suggest that the ENRR over CuFe PBA/f-CNT is mostly controlled by surface adsorption in an acidic electrolyte and by mass transport in an alkaline electrolyte. In situ Raman spectroscopy confirms this finding and shows that the leaching of Fe(CN)6 species from the CuFe PBA/f-CNT composite in an alkaline electrolyte greatly affects the ENRR performance. We believe that the work presented herein offers a new insight into the mechanistic aspects of the ENRR in different electrolyte systems and hence can prove very valuable for the development of effective ENRR electrode/electrolyte systems for practical applications.

Surface modification of carbon nanotubes and their nanocomposites for fuel cell applications: A review

<abstract> <p>Carbon nanotubes (CNTs) have drawn great attention as potential materials for energy conversion and storage systems such as batteries, supercapacitors, and fuel cells. Among these energy conversion and storage systems, the fuel cells had stood out owing to their high-power density, energy conversion efficiency and zero greenhouse gasses emission. In fuel cells, CNTs have been widely studied as catalyst support, bipolar plates and electrode material due to their outstanding mechanical strength, chemical stability, electrical and thermal conductivity, and high specific surface area. The use of CNT has been shown to enhance the electrocatalytic performance of the catalyst, corrosion resistivity, improve the transmission performance of the fuel cell and reduce the cost of fuel cells. The use of CNTs in fuel cells has drastically reduced the use of noble metals. However, the major drawback to the utilization of pristine CNTs in fuel cells are; poor dispersion, agglomeration, and insolubility of CNTs in most solvents. Surface engineering of CNTs and CNT nanocomposites has proven to remarkably remedy these challenges and significantly enhanced the electrochemical performance of fuel cells. This review discusses the different methods of surface modification of CNTs and their nanocomposite utilized in fuel cell applications. The effect of CNTs in improving the performance of fuel cell catalyst, membrane electrode assembly and bipolar plates of fuel cells. The interaction between the CNTs catalyst support and the catalyst is also reviewed. Lastly, the authors outlined the challenges and recommendations for future study of surface functionalized CNTs composite for fuel cell application.</p> </abstract>

Prussian blue-derived FeNx/Fe-based N-doped carbon nanotube catalysts with high ORR electrochemical performance

Prussian blue-derived FeNx/Fe-based N-doped carbon nanotube catalysts with high ORR electrochemical performance.

Bifunctional activity of NiCo on carbon nanotube catalyst for oxygen reduction and evolution on anion exchange membrane-unitized regenerative fuel cells (AEM-URFCs)

Cobalt or nickel/cobalt on carbon nanotube (Co@CNT, NiCo@CNT) catalysts were synthesized by heat treatment of the metal-organic framework, ZIF-67. The X-ray patterns of the synthesized ZIF-67 and the catalysts confirmed their crystalline structures. The field-emission scanning electron microscopy (FESEM) images of ZIF-67 after pyrolysis showed that the dodecahedral crystalline structure of ZIF-67 was retained, and the organic ligands were converted into CNTs and graphitic carbon in an H2 gas environment. The transmission electron microscopy images of the synthesized catalysts confirm the presence of the Ni or Co atoms at the tips of the carbon nanotubes. The NiCo@CNT catalyst exhibited a higher surface area and larger pore volume than those of Co@CNT. X-ray photoelectron spectroscopy confirmed the presence of pyridinic nitrogen, which forms Co-Nx and Ni-Nx, active groups. In anion exchange membrane-unitized regenerative fuel cells (AEM-URFC), NiCo@CNT as a bifunctional oxygen electrode (BOE) catalyst showed a higher current density and lower Tafel slope compared to that of Co@CNT. The cell with the NiCo@CNT catalyst in BOE exhibits the highest current density of 130.0 mA cm−2 at 0.3 V (fuel cell mode) and 32.5 mA cm−2 at 2.0 V (water electrolyzer mode) among all catalysts. This cell exhibited a maximum 38.90 % round trip efficiency, which was comparable to that of the benchmark noble metal (Ir-black) catalyst (38.40 %).

A novel sulfonated solid acid catalyst for efficient conversion of furfuryl alcohol to n-butyl levulinate fuel bioadditive

Furfuryl alcohol (FOL), as a cheap green biomass derivative, has high potential application value in the production of various biomass chemicals. In this work, a sulfonic acid functional porous organic polymer-coated carbon nanotubes catalyst (CNTs@PDVTA-SO3H) was successfully prepared. The structure of as-prepared catalysts was confirmed by TG/DTG, Raman spectra, TEM, FT-IR, N2 adsorption-desorption, and XPS measurements. The catalytic properties of CNTs@PDVTA-SO3H were evaluated for the synthesis of n-butyl levulinate (BL) via the alcoholysis of FOL. It was found that the 99.1 % yield of BL was achieved under optimal conditions. The apparent activation energy of FOL to 2-bufurdoxy methyl-furan (BMF) and BMF to BL was 58.08 kJ/mol and 35.38 kJ/mol, respectively. This work provides a new avenue for the preparation of alkyl levulinates using FOL as raw material.

Synthesis and characterization of enhanced carbon nanotube-supported PtRuBi alloy catalyst for ethylene glycol electrooxidation

At present, carbon nanotube (CNT)-supported Pt-based catalysts are prepared by the sodium borohydride (SBH) method for catalytic ethylene glycol (EG) electrooxidation. The structural and morphology surface analyses of these catalysts are characterized by X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray (SEM-EDX) and mapping analysis, transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS). In addition, the catalytic activity and electrochemical surface area (ECSA) of the catalysts are examined by cyclic voltammetry (CV). The 10 % PtRuBi(80-10-10)/CNT catalyst exhibited the best catalytic activity with a specific activity of 6.10 mA/cm2 and an ECSA value of 488.42 m2/g. Furthermore, it is observed that it exhibited the best stability and resistance by chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) analyses. It can be a promising anode catalyst for direct EG fuel cells.

Peroxymonosulfate-Based Electrochemical Advanced Oxidation: Complication by Oxygen Reduction Reaction.

Peroxymonosulfate (PMS)-based electrochemical advanced oxidation processes (EAOPs) have received widespread attention in recent years, but the precise nature of PMS activation and its impact on the overall process performance remain poorly understood. This study presents the first demonstration of the critical role played by the oxygen reduction reaction in the effective utilization of PMS and the subsequent enhancement of overall pollutant remediation. We observed the concurrent generation of H2O2 via oxygen reduction during the cathodic PMS activation by a model nitrogen-doped carbon nanotube catalyst. A complex interplay between H2O2 generation and PMS activation, as well as a locally increased pH near the electrode due to the oxygen reduction reaction, resulted in a SO4•-/•OH-mixed oxidation environment that facilitated pollutant degradation. The findings of this study highlight a unique dependency between PMS-driven and H2O2-driven EAOPs and a new perspective on a previously unexplored route for further enhancing PMS-based treatment processes.

Dual-core drive hydrogen transfer heterogeneous catalysts based on iridium-enzyme co-modified carbon nanotubes for aromatic aldehyde hydrogenation

Here, a novel dual-core drive transfer hydrogen heterogeneous catalysts Ir@CNT@ADH were developed by sequentially immobilizing the NADH-dependent alcohol dehydrogenase ADH and high active NADH-regenerated catalyst Pyren-Ir1 on the surface of carbon nanotubes. Two pyrene-functionalized Ir-complex, Pyrene-Ir1 and Pyrene-Ir2, were synthesized and demonstrated exceptional coenzyme NADH generation performances, ensuring continuous in situ NADH generation in the enzyme-catalyzed process. These complexes could be easily modified onto carbon nanotube materials by mild π-π conjugated adsorption. The catalysts Ir@CNT@ADH showed significantly improved catalytic hydrogenation efficiency in the reduction of benzaldehyde to the corresponding alcohol, compared to the single Pyrene-Ir1 modified carbon nanotube catalyst Ir@CNT and homogeneous catalytic system in the presence of a catalytic amount of NAD+. The yield of benzyl alcohol increased by more than 50 % in all cases. We assume that the designed dual-core drive heterogeneous catalyst strategy could effectively address the mutual inactivation problem of enzyme and Iridium complexes by blocking the contact between enzymes with Iridium complexes and enhancing the in situ regeneration property of coenzyme NADH in the enzyme catalytic system. Furthermore, catalyst Ir@CNT@ADH also demonstrated excellent catalytic hydrogeneration performances for various substituted benzaldehyde with good recycling performance. Therefore, the proposed dual-core drive heterogeneous catalyst strategy holds great potential for the combined industrial application of chemical catalysts and bioenzymes. It is expected to contribute to the development and preparation of higher-value biochemical products.

Nitrogen incorporation endows copper notable activity for the selective reduction of nitroarenes

The selective reduction of nitroarenes to corresponding anilines using hydrazine hydrate as hydrogen carrier still focus on the employment of Fe and Co based materials as catalysts, while earth abundant and environmental friendly Cu based catalysts are rarely reported in this field. Herein, a cost-effective N modified Cu decorated on carbon nanotubes catalyst CuNCNT was designed and demonstrated comparable or better performance than many reported materials. It showed that N doping notably switched Cu from inactive to highly active, and the atomically dispersed Cu species were proposed to be the critical active sites. Moreover, the in-situ generated active [H] species derived from hydrazine decomposition was considered to be responsible for the reduction.

Carbon nanotubes supported Cs-doped NiFe-layered double hydroxide nanosheets as efficient catalyst for oxygen evolution reaction

The sluggish kinetics of oxygen evolution reaction (OER) becomes one of the big obstacles for large-scale electrolytic water splitting. It is urgent to develop low-cost and efficient OER catalyst with fast reaction kinetics and low energy barrier. Herein, a novel OER catalyst of carbon nanotubes (CNT) supported cesium-doped NiFe-layered double hydroxide nanosheets (Cs–NiFe-LDH) is synthesized via a facile one-pot solvothermal reaction. The porous Cs–NiFe-LDH@CNT catalyst delivers an ultralow overpotential of 240 mV @10 mA cm−2, a small Tafel slope of 56.22 mV dec−1, and excellent long-term stability. The outstanding catalytic performance is mainly attributed to the synergistic effects of Cs doping and CNT network: the modification of the electronic structure of NiFe-LDH by Cs doping and the conductivity enhancement by CNT network can facilitate the electron transfer and kinetics; the Cs doping and CNT introduction can generate rich active sites; the nanoporous architecture can facilitate the ion diffusion and O2 bubble release. This work provides a synergistic strategy to design and synthesize transition metal based OER catalysts with low-cost, fast kinetics and good stability for water splitting.

Bimetallic RuNi Electrocatalyst Coated MWCNTs Cathode for an Efficient and Stable Li-CO2 and Li-CO2 Mars Batteries Performance with Low Overpotential.

Rechargeable lithium-CO2 (Li-CO2 ) batteries are an attractive energy storage technology that can reduce fossil fuel usage and limit the adverse environmental impact of CO2 emissions. However, the high charge overpotential, unstable cycling, and incomplete understanding of the electrochemical process limit its advancement for practical applications. Herein, we develop a Li-CO2 battery by designing a bimetallic ruthenium-nickel catalyst onto multi-walled carbon nanotubes (RuNi/MWCNTs) catalyst as cathode by solvothermal method, which exhibits a lower overpotential of 1.15 V and a discharge capacity of 15,165 mAh g-1 with outstanding coulombic efficiency of 97.4 %. The battery can also operate at high rates and have a stable cycle of more than 80 cycles at a current density of 200 mA g-1 with a fixed 500 mAh g-1 capacity. Furthermore, Mars exploration is made feasible with the Li-CO2 Mars battery composed of the RuNi/MWCNTs as cathode catalyst, which performs very similarly to that of pure CO2 atmosphere. This approach may simplify the process of developing high-performance Li-CO2 batteries to achieve carbon negativity on Earth and for future interplanetary Mars missions.

Elucidating the alloying effect of PdAg/CNT catalysts on formic acid dehydrogenation with kinetic isotope effect

Formic acid has emerged as a promising liquid hydrogen carrier for addressing the transport and storage issues of hydrogen, owing to its high hydrogen density (4.4 wt.%) and easy-handling liquid phase. Therefore, the conversion of formic acid into hydrogen and vice versa is a significant technique. Despite the development of various metal alloy catalysts for efficient production of hydrogen from formic acid, it remains unclear how the metal alloy affects formic acid dehydrogenation from the mechanistic perspective. This study addresses this gap by investigating the alloy effect of Pd and Ag on formic acid dehydrogenation, using PdAg alloy nanoparticles supported on carbon nanotube catalysts. The PdAg alloy, analyzed using multiple characterization methods, demonstrated enhanced catalytic activity, which was maximized at a Pd:Ag molar ratio of 7:3. More importantly, temperature-dependent and kinetic isotope effect experiments demonstrated that the electronic modification of Pd by Ag improved its activity by promoting the cleavage of the CH bond of formic acid. The results suggest that the kinetic isotope effect would be a potent technique for elucidating the alloying effect, as well as providing a deeper understanding of the mechanism.

Electrochemical sensing of caffeic acid antioxidant in wine samples using carbon paste electrode amplified with CdO/SWCNTs

An electrochemical sensor was introduced as an analytical tool for monitoring caffeic acid in food samples. This analytical tool was amplified by cadmium oxide decorated on single wall carbon nanotubes as a new catalyst and showed a powerful ability to sensing of caffeic acid in food products. The presence of cadmium oxide decorated on single wall carbon nanotubes catalyst improved the oxidation signal of caffeic acid about 2.4 times at optimum conditions. The pH investigation confirmed that the redox reaction of caffeic acid was pH dependent and showed maximum sensitivity at pH 7.0. The paste electrode amplified with cadmium oxide decorated on single wall carbon nanotubes was successfully monitored caffeic acid in the concentration range 0.02–200 µM with a detection limit of 9.0 nM, respectively. The standard addition strategy showed a recovery range of 97.96 – 102.59 % to the measurement of caffeic acid in fruit juice, white and red wine that was acceptable for the fabrication of a new analytical tool in food monitoring.

Accelerated Fe(II) regeneration on Fe-doped oxidized carbon nanotube enabling highly-efficient electro-Fenton process for pollutants removal

Heterogeneous electro-Fenton (e-Fenton) is considered a prospective technology for persistent organic contaminants removal that relies on the strongly oxidizing •OH. However, the efficiency of e-Fenton is still low as the sluggish Fe(II) regeneration caused by the difficulty modulating the electronic structure of Fe(III) center. Herein, the Fe-doped oxidized carbon nanotube catalysts (OCNT-Fe) is prepared to accelerate Fe(II) regeneration by introducing electronegative oxygen-containing groups and inducing local charge distribution of Fe(III) center. The OCNT-1Fe exhibits an excellent e-Fenton performance for phenol degradation with a kinetic rate of 0.11 min−1 at pH 6.5, which is the highest among reported state-of-the-art e-Fenton catalysts. Moreover, OCNT-1Fe catalyst is demonstrated to be more energy efficient in coking wastewater treatment with an energy consumption of 6.5 kW h kgCOD−1 than other e-Fenton systems of 9.2–29.7 kW h kgCOD−1. This work offers a new perspective on the high-performance e-Fenton catalysts design by rationally modulating the electronic structure for organic pollutants removal.

Insight into the catalytic mechanism of N-doped carbon nanotubes supported CoNi bimetallic catalyst boosts the base-free oxidative esterification of furfural

Developing active and affordable catalysts for the transformation of biomass platform into value-added chemicals have drawn tremendous attention. Herein, we have successfully prepared a CoNi bimetallic nitrogen-doped carbon nanotubes (CNTs) catalyst, manifesting outstanding catalytic performance for furfural oxidative esterification under base-free conditions (methyl furoate yield >99%), and great recyclability (5th recycle). Importantly, an impressive methyl furoate formation rate of around 1980 μmolmethyl furoate·gcat−1·h−1 was achieved, to the best of our knowledge, the result represents one of the highest methyl furoate formation rates. The results revealed that the doping of Ni into the CoxNiy-CNTs catalyst brings about more active sites. Fourier transforms infrared spectrometer results suggest that the Ni-Nx sites are responsible for the adsorption of the substrate with tilted or vertical adsorption. Furthermore, the introduction of Ni facilitates the improvement of basicity, which in turn promotes the β-hydride elimination. All of these active sites boost the overall activity of the catalyst for the oxidative esterification of furfural.

Elucidation of the electrocatalytic activity origin of Fe3C species and application in the NOx full conversion to valuable ammonia

Conversion of nitrogen oxides (NOx) into ammonia mediated by electrochemical nitrate/nitrite reduction is an ideal route for the treatment and utilization of NOx pollutants. Herein, pure-phase Fe3C nanoparticle supported on N-doped carbon nanotubes catalysts exhibit an NH3 yield rate of 1034.9 μmol h−1 cm−2 with a Faradaic efficiency of 97.3% for NO3− reduction reaction. The excellent intrinsic activity of Fe3C species is attributed to a superior ability to adsorb and activate NO3− and a proper ability to activate protons. The electrochemical in situ FTIR spectra, in situ Raman spectra and density functional theory calculations reveal the reaction process follows a novel multiple reaction pathway. Eventually, a demonstration device is proposed for a full conversion process from NOx to ammonia synthesis including NOx collection, electrochemical nitrate/nitrite reduction and ammonia collection, which made it possible to integrate the green ammonia synthesis system with distributed nitrogen oxide discharge sites.

NiS/MoS2 Anchored Multiwall Carbon Nanotube Electrocatalyst for Hydrogen Generation and Energy Storage Applications

AbstractAlthough sulfide‐based materials are known to be efficient catalysts for oxygen evolution reactions (OER), hydrogen evolution reactions (HER), and supercapacitor applications, improving the catalytic activity of sulfide materials for both electrochemical water splitting and supercapacitors remains a challenging problem. Here, an easy and one‐step integrated methodology is implemented to develop NiS/MoS2 anchored multiwall carbon nanotubes (MWCNT/NiS/MoS2) catalysts that can effectively and robustly catalyze both the HER and OER. The MWCNT/NiS/MoS2 hybrid composite offers the lowest overpotential of 201 mV and 193 mV to achieve a current density of 10 mA/cm2 and −10 mA/cm2 with a small Tafel slope of 58 mV/dec 41 mV/dec for OER and HER, respectively, in alkaline and acidic conditions. The obtained multi‐walled carbon nanotubes anchored with intertwined NiS/MoS2 have a significant number of active sites and defects throughout the structure. The hybrid composite electrode delivered a specific capacitance of >371.45 F/g at 2 A/g in a two‐electrode system, along with high energy density of 51.63 Wh/kg (ED) along with the power density (PD) of 953.63 W/kg, and good capacitance retention ∼82% after 2000 cycles. Therefore, a tandem electron transfer mechanism between NiS and MoS2 (MoS2→NiS) is believed to have an electrical synergistic effect that promotes both HER and OER activity. This research opens a new path for the construction of multi‐component, cheap electrocatalysts that are highly effective for overall water splitting and energy storage applications.

The Semi-Closed Molten Salt-Assisted One-Step Synthesis of N-P-Fe Tridoped Porous Carbon Nanotubes for an Efficient Oxygen Reduction Reaction

Transition metal and heteroatom co-doped carbon nanomaterials (TM-H-C) are considered to be the most promising candidates to replace the expensive platinum-based catalysts for oxygen reduction reactions (ORR). Herein, we report a semi-closed molten salt-assisted one-step strategy for fabricating N-P-Fe-tridoped porous carbon nanotube (CNT) catalysts by using the evaporation of ZnCl2 and the catalysis of iron ions to control the heteroatoms doping process and CNT formation. The resultant densely packed porous CNT structure imparts final catalysts with an enhanced mass transfer ability for electron and ORR-involved species, ensuring a high performance in catalyzing the ORR. Significantly, the optimal catalysts show a half-wave potential of 0.89 V (vs. RHE), which is better than commercial platinum-based catalysts and most reported TM-H-C ORR catalysts. Impressively, when applied to a primary inc-oxygen (Zn–O2) battery, the synthesized catalyst comprehensively outperforms the state-of-the-art platinum-based catalyst in both its maximum power density and specific capacity.

Selective aqueous-phase hydrogenation of furfural to cyclopentanol over Ni-based CNT catalysts

Cyclopentanol (CPL) was an eco-friendly solvent as well as important platform chemical which could be generated from biomass-derived furfural (FFA). In this paper, A series of Ni, Cu, Mo, Co bimetallic catalysts with different metals loadings supported on Carbon nanotubes (CNT) were synthesized by an impregnation method for aqueous-phase hydrogenation of FFA to obtain CPL. Various effects of reaction parameters such as, reaction solvent, reaction temperature, reaction time, and different loading amount of Ni over bimetallic Ni-based CNT catalysts were fully investigated. Among the catalysts studied, (15 + 5) wt% NiCu/CNT catalysts showed high conversion of FFA and 88% selectivity towards CPL in water and 96% selectivity towards acetal in methanol at mild condition of 160℃, 2 MPa hydrogen and 4 h reaction time. NiCu bimetallic synergistic effect was interpreted through H2-TPR and NH3-TPD measurement and a possible pathway was proposed. The features of the CNT supported catalysts were investigated via XRD, XPS, TEM, H2-TPR and NH3-TPD. The Ni and bimetallic NiCu catalysts synthesized in this work were inexpensive and simple, which made them a promising candidate for the conversion of biomass-derived FFA to CPL.

From atomic bonding to heterointerfaces: Co2P/WC constructed by lacunary polyoxometalates induced strategy as efficient hydrogen evolution electrocatalysts at all pH values

Herein, a novel in-situ “atomic binding to heterointerface” strategy is proposed to obtain Co2P/WC@NC/CNTs catalyst with abundant heterointerface between cobalt phosphide and tungsten carbide (Co2P/WC) by the polyoxometalates (POMs)-based metal–organic frameworks (MOFs) precursor. The natural quasi interfaces in K10[Co4(H2O)2(PW9O34)2] molecule crucially guide the abundant Co2P/WC heterointerfaces down to atomic level. Meanwhile, MOFs cages can effectively encapsulate nanosized POMs at molecular level to control the size and dispersion of Co2P/WC nanoparticle, while carbon nanotubes (CNTs) enhance conductivity at nanoscale level. The interfacial electronic modulation between Co2P and WC lowering the energy barrier of the rate determining step, thus Co2P/WC@NC/CNTs showed reasonable hydrogen evolution reaction (HER) activity and stability in all-pH media including sea water. This work provides a “bottom-up” synthetic strategy for confined heterostructures, thus offering the prospect for more efficient interfacial charge modulation.