Dense Pt Nanowire Electrocatalyst for Improved Fuel Cell Performance Using a Graphitic Carbon Nitride-Decorated Hierarchical Nanocarbon Support

An innovative strategy is presented to engineer supported-Pt nanowire (NW) electrocatalysts with a high Pt content for the cathode of hydrogen fuel cells. This involves deposition of graphitic carbon nitride (g-CN) onto 3D multimodal porous carbon (MPC) (denoted as g-CN@MPC) and using the g-CN@MPC as an electrocatalyst support. The protective coating of g-CN on the MPC provides good stability for the electrocatalyst support against electrochemical oxidation, and also enhances oxygen adsorption and provides additional active sites for the oxygen reduction reaction. Compared with commercial carbon black Vulcan XC-72R (denoted as VC) support material, the larger hydrophobic surface area of the g-CN@MPC enables the supported high-content Pt NWs to disperse uniformly on the support. In addition, the unique 3D interconnected pore networks facilitate improved mass transport within the g-CN@MPC support material. As a result, the g-CN@MPC-supported high-content Pt catalysts show improved performance with respect to their counterparts, namely, MPC, VC, and g-CN@VC-supported Pt NW catalysts and the conventional Pt nanoparticle (NP) catalyst (i.e., Pt(20 wt%)NPs/VC (Johnson Matthey)) used as the benchmark. More importantly, the g-CN-tailored high-content Pt NW (≈60 wt%) electrocatalyst demonstrates high PEM fuel cell power/performance at a very low cathode catalyst loading (≈0.1 mg_Pt  cm^-2 ).

Stable and active Pt colloid preparation by modified citrate reduction and a mechanism analysis of inorganic additives

Ultra-small and monodispersed Pt nanoparticles (NPs) have been successfully synthesized in polymer electrolyte membrane fuel cells. The process normally involves the use of capping agents, organic species, templates, and substrates and is thus complex. Hence, obtaining Pt NPs with a clean surface is challenging. In this study, a method for preparing stable and highly dispersed Pt NPs with clean surfaces is proposed. The method involves the use of a modified Na₃C₆H₅O₇ reduction process assisted by NaNO₃ stabilization. The specific complexations of NO₂^- ions possibly alter the reaction kinetics and lower the growth rate of Pt NPs by retarding the reduction reaction. The optimized Pt/carbon nanotube (CNT) catalysts exhibit high mass activity and moderate activity decay after 10,000 times of potential cycling compared with commercially available Pt/C catalysts. Then, membrane electrode assemblies based on the resultant catalysts are characterized. The cell performance of 744 mW cm^-2 (maximum power density) is achieved after the optimized Pt/CNT catalysts are used in carbon black.

A cuboidal [Ni4O4] cluster as a precursor for recyclable, carbon-supported nickel nanoparticle reduction catalysts

A pyridine alkoxide ligated cuboidal [Ni₄O₄] cluster has been selected as the precursor to synthesize carbon-hosted Ni nanoparticles for catalytic conversion of 4-nitrophenol to 4-aminophenol.

Colloidal and micro-carbon spheres derived from low-temperature polymerization reactions

Carbon spheres (CSs) have recently attracted major interest due to their new applications, mainly in energy storage and conversion but also in hard-templating, sorption/catalysis processes, and drug delivery systems. This is attributable to their physico-chemical properties, including their tunable morphology (solid, hollow and core-shell), size, surface area/porosity, good electrical conductivity, low external surface-to-volume ratio, high packing density, enhanced mass transport, robust mechanical stability, low cytotoxicity, and excellent biocompatibility. They can be obtained from a wide variety of carbon precursors and methods. This review covers their production by carbonization of polymer spheres from low-temperature polymerization reactions, considered here as below 250°C. This is a very important method because it allows the synthesis of CSs with different morphologies and doped with other elements or chemical compounds. The preparation of polymer spheres by this technique is well documented in the literature, and the objective of this review is to summarize and give an overview of the most significant publications, proposing a novel classification based on the formation mechanism of the polymer spheres. This classification includes the following polymerization processes: emulsion polymerization and its derivatives, seeded emulsion and inverse emulsion polymerization; precipitation polymerization and its derivative, dispersion polymerization; hard-templating; spray-drying; and hydrothermal or solvothermal treatment of carbohydrates and biomass in general. This review also reports on the morphology and surface characteristics of the CSs obtained by different synthetic approaches. The final section of the review describes the current applications of these CSs, notably in energy storage (supercapacitors and rechargeable batteries) and energy conversion (fuel cells and dye-sensitized solar cells). Besides the numerous applications listed above, they are utilized as sacrificial hard templates to prepare single- and multi-shell hollow spheres of metal oxides and other inorganic compounds and filters, as well as in adsorption and catalysis processes, drug delivery systems, and other minority applications (e.g., lubricants, black pigment in e-papers, and microwave absorber).

The impact of the morphology of the carbon support on the activity and stability of nanoparticle fuel cell catalysts

The key to enhanced fuel cell catalyst durability is a balance of the support's surface area and the respective pore size.

Highly purified CNTs: an exceedingly efficient catalyst support for PEM fuel cell

High performance in PEM fuel cells has been achieved using purified CNTs as catalyst support.

Interface-mediated fabrication of bowl-like and deflated ballon-like hollow carbon nanospheres

In our work, two kinds of hollow carbon nanospheres with controlled morphologies have been successfully prepared from low-cost and nontoxic glucose as the sole carbon precursor under neutral aqueous medium via a simple hydrothermal route. During the process, sodium dodecylbenzene sulfonate (SDBS) and triblock copolymer P123 ((EO)20(PO)70(EO)20) was skillfully selected as the structure-directing agent, respectively. SEM, TEM and AFM results revealed that the two products showed bowl-like and deflated-balloon-like morphology with uniform particle sizes, respectively. Based on the experimental observations, a possible formation mechanism was also discussed, in which the growth of the carbon nanospheres involved an interface-medicated assembly process. The present method was easy, green and mild. Apart from the unique nanostructure, the obtained bowl-like hollow carbon nanospheres exhibited excellent biocompatibility. In particular, it should be mentioned that the open window formed by the bowl-like morphology can facilitate ion transport, thus improving their performances.

Effect of different solvent ratio (ethylene glycol/water) on the preparation of Pt/C catalyst and its activity toward oxygen reduction reaction

Particle size and distribution of 50 wt% Pt/C can be determined by the volume ratio of EG/H₂O in the synthesis.

Hollow-structured mesoporous materials: chemical synthesis, functionalization and applications

Hollow-structured mesoporous materials (HMMs), as a kind of mesoporous material with unique morphology, have been of great interest in the past decade because of the subtle combination of the hollow architecture with the mesoporous nanostructure. Benefitting from the merits of low density, large void space, large specific surface area, and, especially, the good biocompatibility, HMMs present promising application prospects in various fields, such as adsorption and storage, confined catalysis when catalytically active species are incorporated in the core and/or shell, controlled drug release, targeted drug delivery, and simultaneous diagnosis and therapy of cancers when the surface and/or core of the HMMs are functionalized with functional ligands and/or nanoparticles, and so on. In this review, recent progress in the design, synthesis, functionalization, and applications of hollow mesoporous materials are discussed. Two main synthetic strategies, soft-templating and hard-templating routes, are broadly sorted and described in detail. Progress in the main application aspects of HMMs, such as adsorption and storage, catalysis, and biomedicine, are also discussed in detail in this article, in terms of the unique features of the combined large void space in the core and the mesoporous network in the shell. Functionalization of the core and pore/outer surfaces with functional organic groups and/or nanoparticles, and their performance, are summarized in this article. Finally, an outlook of their prospects and challenges in terms of their controlled synthesis and scaled application is presented.

Simple synthesis of mesoporous carbon nanofibers with hierarchical nanostructure for ultrahigh lithium storage

In this study, a simple and reproducible synthesis strategy was developed to fabricate mesoporous carbon nanofibers (MCNFs) by using dual hard templates, a porous anodic aluminum oxide (AAO) membrane, and colloidal silica (Ludox TM-40). By using commercial templates, and removing AAO and the silica simultaneously, the synthesis procedures for MCNFs are greatly simplified without the need for separate preparation or the removal of templates in sequence. With phenol resin as a carbon precursor, the as-prepared MCNFs material reveals not only high surface area and mesoporous volume but also hierarchical nanostructure composed of hollow macrochannels derived from the AAO template, large mesopores (ca. 22 nm) from the removal of silica particles and micropores from the carbonization of phenol resin. Such unique surface and structural characteristics could provide a large quantity of active sites for Li storage and facilitate fast mass transport. Moreover, a one-dimensional (1D) carbon nanofiber (CNF) nanostructure favors fast electron transfer. The as-prepared MCNF anode demonstrates ultrahigh lithium storage capacity particularly at high rates, which is much higher than that reported for the commercial graphite and also significantly higher than other nanostructured carbon materials, such as ordered mesoporous carbon CMK-3 and ordered multimodal porous carbon (OMPC).

Nanoconfinement induced formation of core/shell structured mesoporous carbon spheres coated with solid carbon shell

A novel method for the fabrication of core/shell structured mesoporous carbon spheres with solid shell using a template method has been presented. The unique molecular nanostructures are characterized by XRD, TEM, TGA, and nitrogen adsorption/desorption measurement. The formation mechanism of the mesostructured carbon spheres with a carbon shell is proposed according to the experimental results. Nanoconfinement effect, occurring in the core/shell structured template, is believed to play a key role in mediating the formation of these hierarchical carbon mesostructures, with SnO2 as a template and C2H4 as a carbon source of a mesoporous carbon core. This synthesis method is simple, straightforward, and suitable for the preparation of various nanostructures that are unique scaffolds in catalytic and electrochemical applications.

Porous TiO2 microspheres with tunable properties for photocatalytic air purification

The synthesis of highly-crystalline porous TiO(2) microspheres is reported using ultrasonic spray pyrolysis (USP) in the presence of colloidal silica as a template. We have exploited the interactions between hot SiO(2) template particles surface and TiO(2) precursor that occur during reaction inside the droplets, to control the physical and chemical properties of the resulting particles. Varying the SiO(2) to titanium precursor molar ratio and the colloidal silica dimension, we obtained porous titania microspheres with tunable morphology, porosity, BET surface area, crystallite size, band-gap, and phase composition. In this regard, we have also observed the preferential formation of anatase vs. rutile with increasing initial surface area of the silica template. The porous TiO(2) microspheres were tested in the photocatalytic degradation of nitrogen oxides (NO(x)) in the gas phase. USP prepared nanostructured titania samples were found to have significantly superior specific activity per surface area compared to a commercial reference sample (P25 by Evonik-Degussa).

Nano-architectures of ordered hollow carbon spheres filled with carbon webs by template-free controllable synthesis

Hollow carbon spheres with a carbon network structure inside have been synthesized in the absence of templates for the first time. The samples were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, N(2) adsorption-desorption, and thermogravimetric analyses. The experimental results indicate that the core and the shell of this new structure contain both mesopores and micropores. Furthermore, the inner space and thickness of the hollow carbon spheres can be controlled by adjusting the molar ratio of the glucose and Na(2)SnO(3)·H(2)O. Moreover, a possible formation mechanism has been suggested on the basis of time-dependent experiments. The electrocatalytic activity of methanol oxidation on Pt supported on HC electrocatalyst (Pt/HC@C) is 1.8 times higher than that of Pt supported on commercial Vulcan XC-72 carbon (Pt/C) electrocatalyst at the same Pt loadings.

Different hierarchical nanostructured carbons as counter electrodes for CdS quantum dot solar cells

CdS quantum dot sensitized solar cells based on TiO(2) photoanode and nanostructured carbon as well as Pt as counter electrodes using iodide/triiodide and polysulfide electrolytes were fabricated to improve the efficiency and reduce the cost of solar cells. Compared with conventional Pt (η = 1.05%) and CMK-3 (η = 0.67%) counter electrodes, hollow core-mesoporous shell carbon (HCMSC) counter electrode using polysulfide electrolyte exhibits much larger incident photon to current conversion efficiency (IPCE = 27%), photocurrent density (J(sc) = 4.31 mA.cm(-2)) and power conversion efficiency (η = 1.08%), which is basically due to superb structural characters of HCMSC such as large specific surface area, high mesoporous volume, and 3D interconnected well-developed hierarchical porosity network, which facilitate fast mass transfer with less resistance and enable HCMSC to have highly enhanced catalytic activity toward the reduction of electrolyte shuttle.

Incorporating hierarchical nanostructured carbon counter electrode into metal-free organic dye-sensitized solar cell

Hierarchical nanostructured carbon with a hollow macroporous core of ca. 60 nm in diameter in combination with mesoporous shell of ca. 30 nm in thickness has been explored as counter electrode in metal-free organic dye-sensitized solar cell. Compared with other porous carbon counterparts such as activated carbon and ordered mesoporous carbon CMK-3 and Pt counter electrode, the superior structural characteristics including large specific surface area and mesoporous volume and particularly the unique hierarchical core/shell nanostructure along with 3D large interconnected interstitial volume guarantee fast mass transport in hollow macroporous core/mesoporous shell carbon (HCMSC), and enable HCMSC to have highly enhanced catalytic activity toward the reduction of I(3)(-), and accordingly considerably improved photovoltaic performance. HCMSC exhibits a V(oc) of 0.74 V, which is 20 mV higher than that (i.e., 0.72 V) of Pt. In addition, it also demonstrates a fill factor of 0.67 and an energy conversion efficiency of 7.56%, which are markedly higher than those of its carbon counterparts and comparable to that of Pt (i.e., fill factor of 0.70 and conversion efficiency of 7.79%). Furthermore, HCMSC possesses excellent chemical stability in the liquid electrolyte containing I(-)/I(3)(-) redox couples, namely, after 60 days of aging, ca. 87% of its initial efficiency is still achieved by the solar cell based on HCMSC counter electrode.