Siguo Chen

Space‐Confinement‐Induced Synthesis of Pyridinic‐ and Pyrrolic‐Nitrogen‐Doped Graphene for the Catalysis of Oxygen Reduction

The development of high-performance and low-cost catalytic materials for the oxygen reduction reaction (ORR) has been a major challenge for the large-scale application of fuel cells. Currently, platinum and platinum-based alloys are the most efficient ORR catalysts in fuel-cell cathodes; however, they cannot meet the demand for the widespread commercialization of fuel cells because of the scarcity of platinum. Thus, the ongoing search for platinum-free catalysts for the ORR has attracted much attention. Graphene, single-layer sheets of sp-hybridized carbon atoms, has attracted tremendous attention and research interest. The abundance of free-flowing p electrons in carbon materials composed of sp-hybridized carbon atoms makes these materials potential catalysts for reactions that require electrons, such as the ORR. However, these p electrons are too inert to be used directly in the ORR. In N-doped electron-rich carbon nanostructures, carbon p electrons have been shown to be activated through conjugation with lone-pair electrons from N dopants; thus, O2 molecules are reduced on the positively charged C atoms that neighbor N atoms. Recently, Hu and co-workers found that as long as the electroneutrality of the sp-hybridized carbon atoms is broken and charged sites that favor O2 adsorption are created, these materials will be transformed into active metal-free ORR electrocatalysts regardless of whether the dopants are electron-rich (e.g., N) or electrondeficient (e.g., B). Nitrogen-doped carbon (NC) materials are considered to be promising catalysts because of their acceptable ORR activity, low cost, good durability, and environmental friendliness. However, their ORR activity is less competitive, especially in acidic media. Relative to commercial Pt/C, the difference in the half-wave potential for ORR is within 25 mV in alkaline electrolytes but is greater than 200 mV in acidic electrolytes. The activity of NC materials can be enhanced through efficient N doping with sufficient active species that favor ORR and through an increase in electrical conductivity. The annealing of graphitized carbon materials, such as carbon nanotubes and microporous carbon black, in NH3 leads to insufficient substitution of nitrogen because of the well-ordered structure of the host materials. Alternatively, the direct pyrolysis of nitrogen-containing hydrocarbons or polymers produces NC materials with good incorporation of nitrogen. However, suitable pyrolysis temperatures are difficult to pinpoint; without optimization, temperatures that are excessively low or excessively high lead to low electronic conductivity or a remarkable loss of active N species, respectively. Recently, mesoporous-alumina-assisted and silica-template-assisted nitrogen incorporation, which can preserve a high content of N in synthesized NC materials, have been reported. However the activities of the resulting NC materials in the ORR were still significantly lower than that of Pt/C, even when the N content was as high as 10.7 atm%. Among three types of N atoms, that is, pyridinic, pyrrolic, and quaternary N, only the pyridinic and pyrrolic forms, which have planar structures, have been proven to be active in the ORR. In contrast, quaternary N atoms, which possess a 3D structure, are not active in the ORR. The low electrical conductivity of NC materials with quaternary N atoms results from the interruption of their p–p conjugation by the 3D structure and is thought to be predominantly responsible for the poor catalysis. Therefore, the synthesis of NC materials with more planar pyridinic and pyrrolic N atoms and fewer quaternary N atoms is important for the preparation of ORR-active catalysts. Herein, we present a novel strategy for the selective synthesis of pyridinicand pyrrolic-nitrogen-doped graphene (NG) by the use of layered montmorillonite (MMT) as a quasi-closed flat nanoreactor, which is open only along the perimeter to enable the entrance of aniline (AN) monomer molecules. The flat MMT nanoreactor, which is less than 1 nm thick, extensively constrains the formation of quaternary N because of its 3D structure but facilitates the formation of pyridinic and pyrrolic N. Nitrogen is well-known to be incorporated into quaternary N in tetrahedral sp hybridization but incorporated into pyridinic and pyrrolic N in planar sp hybridization. The confinement effect of MMT ensures that N is incorporated into the structure and that the graphitization is successful without significant loss of N species. Furthermore, planar pyridinic and pyrrolic N can be [*] Dr. W. Ding, Prof. Z.-D. Wei, Dr. S.-G. Chen, Dr. X.-Q. Qi, Dr. T. Yang, Dr. S. F. Alvi, Dr. L. Li The State Key Laboratory of Power Transmission Equipment and System Security and New Technology, College of Chemistry and Chemical Engineering, Chongqing University Shapingba 174, Chongqing (China) E-mail: zdwei@cqu.edu.cn

Nanostructured Polyaniline-Decorated Pt/C@PANI Core–Shell Catalyst with Enhanced Durability and Activity

We have designed and synthesized a polyaniline (PANI)-decorated Pt/C@PANI core-shell catalyst that shows enhanced catalyst activity and durability compared with nondecorated Pt/C. The experimental results demonstrate that the activity for the oxygen reduction reaction strongly depends on the thickness of the PANI shell and that the greatest enhancement in catalytic properties occurs at a thickness of 5 nm, followed by 2.5, 0, and 14 nm. Pt/C@PANI also demonstrates significantly improved stability compared with that of the unmodified Pt/C catalyst. The high activity and stability of the Pt/C@PANI catalyst is ascribed to its novel PANI-decorated core-shell structure, which induces both electron delocalization between the Pt d orbitals and the PANI π-conjugated ligand and electron transfer from Pt to PANI. The stable PANI shell also protects the carbon support from direct exposure to the corrosive environment.

An extraordinarily stable catalyst: Pt NPs supported on two-dimensional Ti3C2X2 (X = OH, F) nanosheets for oxygen reduction reaction

High dispersion Pt nanoparticles supported on 2D Ti3C2X2 (X = OH, F) nanosheets are presented and electro-chemical measurements confirm that the Pt/Ti3C2X2 catalyst shows enhanced durability and improved ORR activity compared with the commercial Pt/C catalyst.

Ni-doped Mo2C nanowires supported on Ni foam as a binder-free electrode for enhancing the hydrogen evolution performance

In this study, an inexpensive electrocatalyst, Ni-doped Mo2C nanowires, were grown directly on Ni foam via a hydrothermal reaction combined with a carburization process. X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and linear scanning voltammetry (LSV) were used to scrutinize the catalysts and their electrochemical performance. The results showed that the designed NiMo2C/NF catalyst displays enhanced catalytic activity toward hydrogen production with a low onset overpotential of 21 mV. For driving a cathodic current density of 100 mA cm−2, it only needs an overpotential of 150 mV. Such excellent performance of NiMo2C/NF could be ascribed to the high intrinsic activity from a synergistic function of Ni and Mo2C, as well as to the exposure of more Ni-doped Mo2C sites provided by the high aspect ratio of a one-dimensional (1D) structure and rich surface area.

Enhanced dispersion and durability of Pt nanoparticles on a thiolated CNT support

High dispersion Pt nanoparticles supported on surface thiolation functional carbon nanotubes (SH-CNTs) is presented and electrochemical measurements confirm that the Pt/SH-CNTs catalyst shows good durability and excellent ORR activity.

A Strategy to Promote the Electrocatalytic Activity of Spinels for Oxygen Reduction by Structure Reversal

The electrocatalytic performance of a spinel for the oxygen reduction reaction (ORR) can be significantly promoted by reversing its crystalline structure from the normal to the inverse. As the spinel structure reversed, the activation and cleavage of O-O bonds are accelerated owing to a dissimilarity effect of the distinct metal atoms co-occupying octahedral sites. The Co(II)Fe(III)Co(III)O4 spinel with the Fe and Co co-occupying inverse structure exhibits an excellent ORR activity, which even exceeds that of the state-of-the-art commercial Pt/C by 42 mV in alkaline medium.

Structural Evolution of Solid Pt Nanoparticles to a Hollow PtFe Alloy with a Pt‐Skin Surface via Space‐Confined Pyrolysis and the Nanoscale Kirkendall Effect

A space-confined interfacial conversion approach is developed to directly transform 3 nm solid Pt nanoparticles into a 5 nm hollow PtFe alloy featuring a Pt-skin surface. The approach presented for the structural evolution from solid Pt NPs to hollow PtFe alloy with controlled size, structure, and composition can be applied to other multimetallic electrocatalysts.

Pd-induced Pt(IV) reduction to form Pd@Pt/CNT core@shell catalyst for a more complete oxygen reduction†

We describe a facile and controllable process for preparing Pd@Pt/CNT core@shell catalysts for the oxygen reduction reaction (ORR) via Pd-induced Pt(IV) reduction on Pd/CNT. The mass-specific activity for the ORR of the Pd@Pt/CNT catalysts is 7–9 times higher than that of the state-of-the-art Pt/C catalysts, but the yield of H2O2, a harmful species for the stability of catalysts, of the former is only 14.1% of that of the latter. The reason for the enhanced activity and the lower H2O2 yield on the Pd@Pt/CNT catalysts was studied by DFT calculations.

A catalyst superior to carbon-supported-platinum for promotion of the oxygen reduction reaction: reduced-polyoxometalate supported palladium

The high solubility of polyoxometalates (POMs) in basic electrolytes limits their practical applications. Fortunately, we found that this shortcoming can be overcome by electrochemically reducing the POMs. The reduced POMs (rPOM) are extremely stable in alkaline solutions and can participate in electrocatalytic cycles. Herein, we present a novel rPOM as a Pd-catalyst-substrate to achieve a high catalytic performance towards the oxygen reduction reaction (ORR). Interestingly, although rPOM alone has very low catalytic activity, the Pd/rPOM hybrid catalyst exhibits excellent electrocatalytic performance for ORR in alkaline media, even better than the commercial Pt/C catalyst. The much improved ORR activity of Pd/rPOM is based on electron delocalization between the Pd and the rPOM support, causing a down-shift in the d-band structure of the Pd NPs. Furthermore, the rPOM in the Pd–rPOM hybrid serves as an assistant catalyst, facilitating the decomposition of the harmful hydrogen peroxide intermediates. The method reported here will promote broader interest in the further development of other new catalysts for real-world applications.

Sn and Sb co-doped RuTi oxides supported on TiO2 nanotubes anode for selectivity toward electrocatalytic chlorine evolution

The (Ru0.3Ti0.34Sn0.3Sb0.06)O2–TiO2 nanotubes (TNTs) anode has been prepared via anodization, deposition, and annealing. X-ray diffraction, field-emission scanning electron microscopy, cyclic voltammetry, and linear scanning voltammetry were used to scrutinize the electrodes and the electrochemical activity. The results indicate that highly ordered TNTs with large specific surface area could be implanted with active metal oxides. The catalyst firmly binds with the TNTs and enhances the electrochemical stability of the electrode. It displays high over-potential for oxygen evolution reaction. Accordingly, the constructed (Ru0.3Ti0.34Sn0.3Sb0.06)O2–TNTs anode exhibits a greater potential difference (ΔE) between the evolutions of oxygen and chlorine than that exhibited by the traditional dimensionally stable anode, which is beneficial for improving the selectivity toward chlorine evolution reaction. This superior performance is explained in terms of the surface properties and geometric structure of coated catalyst, as well as the electrochemical selectivity ascribed by the addition of tin and antimony species.