Shengli Chen

Fe–N doped carbon nanotube/graphene composite: facile synthesis and superior electrocatalytic activity

Low cost alternatives to the expensive and scarce Pt to catalyze the oxygen reduction reaction (ORR) in acid media are essential for the proton-exchange-membrane (PEM) fuel cells to become economically viable. Chemically doped nanocarbons are among the most promising candidates in this regard. We report the facile synthesis and superior electrocatalytic activity of an Fe–N doped nanocarbon composite of carbon nanotubes (CNTs) grown on/between graphene sheets. The structure and composition of the composite is characterized by using a variety of techniques including SEM, TEM, N2 adsorption/desorption isotherms, XPS, XRD, and Mossbauer spectroscopy. It is shown that the in situ growth of CNTs in the presence of graphene sheets not only produces a tubes-on/between-sheets architecture that enhances the dispersion of CNTs and graphene sheets, but also leads to optimized doping and coordination of nitrogen and Fe which favour the ORR. The composite can catalyze the ORR much more efficiently than either of the single materials containing only CNTs or graphene synthesized under similar conditions, and similarly to Pt/C in both alkaline and acid media.

Three-dimensional ordered macroporous IrO2 as electrocatalyst for oxygen evolution reaction in acidic medium

Three-dimensional ordered macroporous (3-DOM) IrO2 was synthesized by using the silica colloidal crystal template method and explored as electrocatalyst for oxygen evolution reaction (OER) in acidic medium. X-Ray diffraction (XRD) patterns indicate that the prepared 3-DOM IrO2 has a rutile structure. Images of scanning and transmission electron microscopies suggest that the 3-DOM IrO2 possesses a hierarchical pore structure, in which the honeycomb array of 300 nm primary macropores were cross-linked by secondary mesopores on the walls. The presence of mesopores is also indicated by the N2 adsorption/desorption isotherms and the small-angle XRD patterns. As compared with IrO2 prepared by conventional colloidal method, the 3-DOM IrO2 exhibited much larger BET area and voltammetric charges. Accordingly, about two and half times enhancement in OER activity was achieved by using 3-DOM IrO2 as the electrode materials, showing prospect of 3-DOM materials in reducing the demand of the expensive IrO2 electrocatalyst for OER in water electrolysis.

A novel mediatorless microbial fuel cell based on direct biocatalysis of Escherichia coli

A mediatorless microbial fuel cell based on the direct biocatalysis of Escherichia coli shows significantly enhanced performance by using bacteria electrochemically-evolved in fuel cell environments through a natural selection process and a carbon/PTFE composite anode with an optimized PTFE content.

Graphene Nanoelectrodes: Fabrication and Size-Dependent Electrochemistry

The fabrication and electrochemistry of a new class of graphene electrodes are presented. Through high-temperature annealing of hydrazine-reduced graphene oxides followed by high-speed centrifugation and size-selected ultrafiltration, flakes of reduced graphene oxides (r-GOs) of nanometer and submicrometer dimensions, respectively, are obtained and separated from the larger ones. Using n-dodecanethiol-modified Au ultramicroelectrodes of appropriately small sizes, quick dipping in dilute suspensions of these small r-GOs allows attachment of only a single flake on the thiol monolayer. The electrodes thus fabricated are used to study the heterogeneous electron transfer (ET) kinetics at r-GOs and the nanoscopic charge transport dynamics at electrochemical interfaces. The r-GOs are found to exhibit similarly high activity for electrochemical ET reactions to metal electrodes. Voltammetric analysis for the relatively slow ET reaction of Fe(CN)6(3-) reduction produces slightly higher ET rate constants at r-GOs of nanometer sizes than at large ones. These ET kinetic features are in accordance with the defect-dominant nature of the r-GOs and the increased defect density in the nanometer-sized flakes as revealed by Raman spectroscopic measurements. The voltammetric enhancement and inhibition for the reduction of Ru(NH3)6(3+) and Fe(CN)6(3-), respectively, at r-GO flakes of submicrometer and nanometer dimensions upon removal of supporting electrolyte are found to significantly deviate in magnitude from those predicted by the electroneutrality-based electromigration theory, which may evidence the increased penetration of the diffuse double layer into the mass transport layer at nanoscopic electrochemical interfaces.

An Fe–N–C hybrid electrocatalyst derived from a bimetal–organic framework for efficient oxygen reduction

A simple Zn/Fe bimetallic zeolitic–imidazolite framework (ZIF) carbonization method is developed to synthesize an Fe–N–C hybrid with hierarchical nitrogen-doped porous carbons crossed by carbon nanotubes. Both the specific ratios of Zn/Fe in the bimetallic metal–organic framework (MOF) precursors and the selected annealing temperature are essential for the formation of this unique hybrid structure with good conductivity and exposure of more active sites. The resulting FeNC-20-1000 hybrid electrocatalyst exhibits excellent oxygen reduction reaction (ORR) activity, with a half-wave potential of 0.770 V comparable to that of the commercial Pt/C catalysts in acidic media, and a half-wave potential of 0.880 V, ca. 50 mV more positive than that of Pt/C for ORR in alkaline solution. More importantly, the as-prepared Fe–N–C hybrid exhibits much more stability for the ORR in both acidic and alkaline solutions than Pt/C, which makes it among the best non-noble-metal catalysts ever reported for ORR under acidic and alkaline conditions.

Synergistic increase of oxygen reduction favourable Fe–N coordination structures in a ternary hybrid of carbon nanospheres/carbon nanotubes/graphene sheets

A Fe/N co-doped ternary nanocarbon hybrid, with uniform bamboo-like carbon nanotubes (CNTs) in situ grown on/between the single/few-layer graphene sheets interspaced by carbon nanosphere aggregates, was prepared through a one-pot heat treatment of a precursor mixture containing graphene oxide, Vulcan XC-72 carbon nanospheres, nitrogen rich melamine and small amounts of Fe ions. Physical characterization including electron microscopic images, N2 adsorption-desorption isotherms, pore size distribution, XPS, XRD, Mössbauer spectra, and EDX revealed that the 0-D/1-D/2-D ternary hybrid architecture not only offered an optimized morphology for high dispersion of each nanocarbon moiety, while the carbon nanosphere interspaced graphene sheets have provided a platform for efficient reaction between Fe ions and melamine molecules, resulting in uniform nucleation and growth of CNTs and formation of high density Fe-N coordination assemblies that have been believed to be the active centers for the oxygen reduction reaction (ORR) in carbon-based nonprecious metal electrocatalysts. In the absence of graphene oxides or carbon nanospheres, a similar heat treatment was found to result in large amounts of elemental Fe and Fe carbides and entangled CNTs with wide diameter distributions. As a result, the ternary Fe/N-doped nanocarbon hybrid exhibits ORR activity much higher than the Fe-N doped single or binary nanocarbon materials prepared under similar heat treatment conditions, and approaching that of the state-of-the-art carbon-supported platinum catalyst (Pt/C) in acidic media, as well as superior stability and methanol tolerance to Pt/C.

A cobalt-based hybrid electrocatalyst derived from a carbon nanotube inserted metal–organic framework for efficient water-splitting

The design of high-efficiency non-precious electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is extremely important for developing sustainable energy technologies. Herein, we report a metal–organic framework (MOF) template carbonization route to prepare a Co-based hybrid electrocatalyst with superior performance for both HER and OER in an alkaline solution. Starting from a carbon nanotube (CNT) inserted Co MOF, the Co-NC/CNT hybrid with a N-doped carbon nanoframe composed of hollow Co nanoparticles interlinked by CNTs is first synthesized via carbonization. This unique 3D structure is shown to have highly exposed active sites and good conductivity. Most importantly, the Co-NC/CNT hybrid can act as bifunctional catalysts on both anode and cathode for electrochemical water splitting, to deliver a current density of 10 mA cm−2 under a cell voltage as low as 1.625 V.

Twinned growth behaviour of two-dimensional materials

Twinned growth behaviour in the rapidly emerging area of two-dimensional nanomaterials still remains unexplored although it could be exploited to fabricate heterostructure and superlattice materials. Here we demonstrate how one can utilize the twinned growth relationship between two two-dimensional materials to construct vertically stacked heterostructures. As a demonstration, we achieve 100% overlap of the two transition metal dichalcogenide layers constituting a ReS2/WS2 vertical heterostructure. Moreover, the crystal size of the stacked structure is an order of magnitude larger than previous reports. Such twinned transition metal dichalcogenides vertical heterostructures exhibit great potential for use in optical, electronic and catalytic applications. The simplicity of the twinned growth can be utilized to expand the fabrication of other heterostructures or two-dimensional material superlattice and this strategy can be considered as an enabling technology for research in the emerging field of two-dimensional van der Waals heterostructures.

Tailoring molecular architectures of Fe phthalocyanine on nanocarbon supports for high oxygen reduction performance

Cost-effective non-precious metal electrocatalysts for the oxygen reduction reaction (ORR) is the key for fuel cells to become a viable electricity generation technology. Metal macrocyclic compounds such as Fe/Co porphyrins and phthalocyanines are excellent molecular catalysts for O2 reduction; but they are still considerably less competitive than Pt-based materials when catalyzing the ORR in electrochemical environments. Using Fe phthalocyanine (FePc) as a model compound, we show that the electrocatalytic activity of metal macrocyclic compounds for the ORR can be greatly enhanced through tailoring assembling architectures on high-surface-area nanocarbons. By simply ball-milling FePc with nanocarbons, such as graphene nanosheets and carbon-black nanoparticles, molecular architectures of FePc from nanorods to uniform thin shells are obtained. The resulting carbon-supported FePc composites exhibit ORR performance much superior to the state-of-the-art carbon-supported Pt in alkaline solution, with up to a 60 mV positive shift in the half-wave potential and more than 5 times increase in the mass activity. As well as showing that the molecule–support interaction provides a degree of control on the molecular architectures of metal macrocyclic compounds, the present work reveals that the FePc molecule is intrinsically much more efficient than Pt in catalyzing the ORR in alkaline media, and therefore has great prospects as a cathode electrocatalyst in alkaline fuel cells.

Oxygen Reduction Electrocatalyst of Pt on Au Nanoparticles through Spontaneous Deposition

A straightforward one-step spontaneous deposition approach for growth of Pt atomic shell on Au nanoparticles and the superior activity and durability of the resulted Pt-on-Au nanoparticles for the oxygen reduction reaction (ORR) are reported. Transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive spectrometry, and electrochemical measurements indicate that Pt can be spontaneously deposited on Au surface upon simply dispersing carbon-supported Au nanoparticles in PtCl42–-containing solution, without introducing any extraneous reducing agents or any pre/post-treatments. The deposited Pt atoms are uniformly distributed on the surface of Au nanoparticles, with coverage tunable by the concentration of PtCl42– and temperatures. An approximate monolayer of Pt forms at temperature of ca. 80 °C and PtCl42– concentrations of above 10–4 mol/L. The obtained Pt-on-Au core–shell nanoparticles catalyze the ORR with specific and mass activities of Pt that are 3.5 times higher than that of pure Pt nanoparticles. Moreover, they exhibit no visible activity degradation after undergoing long-term oxidization/reduction cycling in O2-saturated acid media, therefore showing great prospect as durable cathode electrocatalysts in proton-exchange membrane fuel cells.