Transition metals (Fe, Co, and Ni) encapsulated in nitrogen-doped carbon nanotubes as bi-functional catalysts for oxygen electrode reactions

MOF-derived MoC-Fe heterojunctions encapsulated in N-doped carbon nanotubes for water splitting

Nitrogen-doped and undoped carbon nanotubes (CNTs) were synthesized from ferrocene, nickelocene, and cobaltocene metal catalysts. Electrochemical testing for an oxygen reduction reaction (ORR) showed that nitrogen-doped CNTs synthesized from ferrocene had improved catalytic activity while nanotubes synthesized from nickelocene and cobaltocene, doped with a comparable amount of nitrogen and having similar stacked-cups structure as nitrogen doped CNTs from ferrocene, had a performance only slightly better than that of undoped CNTs. Ferrocene-based nitrogen-doped CNTs also demonstrated similar long-term stability and higher CO tolerance compared to Pt/C catalyst. Detailed ORR mechanisms were also studied and carbon nanomaterials showed different ORR processes as a result of the metal catalyst utilized in the chemical synthesis. Nitrogen-doped and undoped CNTs synthesized from nickelocene show a preferential 4-electron process as compared to materials synthesized from ferrocene and cobaltocene. We believe that the metal used in the growth process regulates the mechanism of oxygen reduction and can be used to develop improved nitrogen-doped carbon nanomaterials as nonprecious-metal catalysts for fuel cells.

Energetics and electronic structures of pyridine-type defects in nitrogen-doped carbon nanotubes

Growth of carbon nanotubes and nitrogen-doped carbon nanotubes on “carbon nanotube seeds”

Iron nanoparticles in aligned arrays of pure and nitrogen-doped carbon nanotubes

A series of hybrids of nitrogen-doped graphitic porous carbon and carbon nanotubes (NGPC/NCNTs) are readily prepared in a stepwise manner by using a typical metal–organic framework (MOF-5) and urea as the carbon and nitrogen precursors, and nickel as the graphitization catalyst, respectively. These NGPC/NCNTs hybrids have demonstrated prominent catalytic activities toward oxygen reduction reaction (ORR) in alkaline medium. Compared to the benchmark Pt/C catalyst, the optimized NGPC/NCNT-900 (annealed at 900 °C) exhibits superior catalytic activity, durability and methanol tolerance, which makes it one of the best ORR electrocatalysts derived from MOFs. The promising properties in NGPC/NCNT-900 are mainly attributed to synergistic contributions of its unique hybrid structure, rich nitrogen doping, high graphitic degree, and large surface area. This attractive route for the preparation of NGPC/NCNTs holds promise for general use of a great number of available and yet rapidly growing MOFs in constructing hig...

PtRu nanoparticles supported on nitrogen-doped multiwalled carbon nanotubes as catalyst for methanol electrooxidation

Heat treating nitrogen-doped multiwalled carbon nanotubes containing up to six different types of nitrogen functionalities transforms particular nitrogen functionalities into other types which are more catalytically active toward oxygen reduction reactions (ORR). In the first stage, the unstable pyrrolic functionalities transform into pyridinic functionalities followed by an immediate transition into quaternary center and valley nitrogen functionalities. By measuring the electrocatalytic oxidation reduction current for the different samples, we achieve information on the catalytic activity connected to each type of nitrogen functionality. Through this, we conclude that quaternary nitrogen valley sites, N-Q(valley), are the most active sites for ORR in N-CNTs. The number of electrons transferred in the ORR is determined from ring disk electrode and rotating ring disk electrode measurements. Our measurements indicate that the ORR processes proceed by a direct four-electron pathway for the N-Q(valley) and the pyridinic sites while it proceeds by an indirect two-electron pathway via hydrogen peroxide at the N-Q(center) sites. Our study gives both insights on the mechanism of ORR on different nitrogen functionalities in nitrogen-doped carbon nanostructures and it proposes how to treat samples to maximize the catalytic efficiency of such samples.

Encapsulated Fe3C boosted electrocatalytic performance for oxygen reduction reaction of N-doped carbon nanotube

In the past decade, tremendous efforts have been devoted to the search for the alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. Recently, metal-nitrogen-carbon (M-N-C) systems, especially 3d transition metals (TM) and their alloys encapsulated in nitrogen-doped carbon based materials (TM@N-C), have attracted increasing attention due to their low cost and high ORR activity. Here, a simple and novel strategy is developed to synthesize sandwich-structured TM@N-C composites, in which ultrafine Fe nanoparticles are encapsulated in nitrogen-doped carbon nanotubes (N-CNTs) grafted on both sides of reduced graphene oxide (rGO) sheets by pyrolysis of ammonium ferric citrate-functionalized zeolitic imidazolate framework-8@graphene oxide (Fe@ZIF-8@GO). The resulting Fe@N-CNTs@rGO composites naturally integrate zero-dimensional (0D) Fe nanoparticles, one-dimensional (1D) N-CNTs, and two-dimensional (2D) graphene into a three-dimensional (3D) hierarchical architecture with highly dispersed active sites, a large surface area, and abundant porosity. Because of these structural advantages, the sandwich-structured Fe@N-CNTs@rGO composites display a half-wave potential of 0.83 V in a 0.1 M KOH solution for the ORR, comparable to that of commercial Pt/C catalysts, and more excellent durability and resistance to fuel molecules. The proposed strategy paves a new way for the synthesis of non-precious high-performance electrocatalysts for energy conversion applications.

Carbon-based nanomaterials have been the focus of research interests in the past 30 years due to their abundant microstructures and morphologies, excellent properties, and wide potential applications, as landmarked by 0D fullerene, 1D nanotubes, and 2D graphene. With the availability of high specific surface area (SSA), well-balanced pore distribution, high conductivity, and tunable wettability, carbon-based nanomaterials are highly expected as advanced materials for energy conversion and storage to meet the increasing demands for clean and renewable energies. In this context, attention is usually attracted by the star material of graphene in recent years. In this Account, we overview our studies on carbon-based nanotubes to nanocages for energy conversion and storage, including their synthesis, performances, and related mechanisms. The two carbon nanostructures have the common features of interior cavity, high conductivity, and easy doping but much different SSAs and pore distributions, leading to different performances. We demonstrated a six-membered-ring-based growth mechanism of carbon nanotubes (CNTs) with benzene precursor based on the structural similarity of the benzene ring to the building unit of CNTs. By this mechanism, nitrogen-doped CNTs (NCNTs) with homogeneous N distribution and predominant pyridinic N were obtained with pyridine precursor, providing a new kind of support for convenient surface functionalization via N-participation. Accordingly, various transition-metal nanoparticles were directly immobilized onto NCNTs without premodification. The so-constructed catalysts featured high dispersion, narrow size distribution and tunable composition, which presented superior catalytic performances for energy conversions, for example, the oxygen reduction reaction (ORR) and methanol oxidation in fuel cells. With the advent of the new field of carbon-based metal-free electrocatalysts, we first extended ORR catalysts from the electron-rich N-doped to the electron-deficient B-doped sp2 carbon. The combined experimental and theoretical study indicated the ORR activity originated from the activation of carbon π electrons by breaking the integrity of π conjugation, despite the electron-rich or electron-deficient nature of the dopants. With this understanding, metal-free electrocatalysts were further extended to the dopant-free defective carbon nanomaterials. Moreover, we developed novel 3D hierarchical carbon-based nanocages by the in situ MgO template method, which featured coexisting micro-meso-macropores and much larger SSA than the nanotubes. The unique 3D architecture avoids the restacking generally faced by 2D graphene due to the intrinsic π-π interaction. Consequently, the hierarchical nanocages presented superior performances not only as new catalyst supports and metal-free electrocatalysts but also as electrode materials for energy storage. State-of-the-art supercapacitive performances were achieved with high energy density and power density, as well as excellent rate capability and cycling stability. The large interior space of the nanocages enabled the encapsulation of high-loading sulfur to alleviate polysulfide dissolution while greatly enhancing the electron conduction and Li-ion diffusion, leading to top level performance of lithium-sulfur battery. These results not only provide unique carbon-based nanomaterials but also lead to in-depth understanding of growth mechanisms, material design, and structure-performance relationships, which is significant to promote their energy applications and also to enrich the exciting field of carbon-based nanomaterials.

Ultrasonic cavitation assisted deposition of catalytically active metals on nitrogen-doped and non-doped carbon nanotubes — A comparative study

Density functional theory study of N-doping effect on the stability and activity of Pd/NCNT catalysts for heck reaction

Detailed studies of mechanisms for hydrogen dissociative adsorption and diffusion on pure and nitrogen-doped (8, 0) carbon nanotubes are carried out using the first-principles density functional theory method. (1) For pure carbon nanotubes, we have identified the energetically most favorable dissociative pathway for hydrogen adsorption, with a barrier height of $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We also found that the adsorbed hydrogen atoms can act as an autocatalyst for further dissociative adsorption of hydrogen molecules. (2) It is found that on pure carbon nanotubes the diffusion of hydrogen atoms is constrained by interaction with neighboring adsorbed hydrogen atoms. The diffusion barrier is around $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for an isolated hydrogen atom, but becomes substantially higher at around $1.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the presence of adsorbed hydrogen in neighboring positions. (3) Doping the nanotube with nitrogen considerably alters the catalytic effects of the carbon nanotube for hydrogen dissociative adsorption. The dissociative adsorption of hydrogen on the carbon nanotube is greatly enhanced, with the barrier substantially reduced to ca. $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The differences in the barrier heights are explained through analysis of the electronic structure changes of the nanotube.

Green synthesis of transition metal nanocrystals encapsulated into nitrogen-doped carbon nanotubes for efficient carbon dioxide capture