Forced obedience: Layer‐structured montmorillonite (MMT) was used as a nanoreactor for the generation of planar pyridinic and pyrrolic N sites in nitrogen‐doped graphene (NG; see picture). The selectivity for the formation of planar N sites was inversely proportional to the interspace width (δ) of the MMT and reached a maximum of 90.27 %. The NG catalyst exhibited low electrical resistance, high electrocatalytic activity, and good stability.

ABSTRACT: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 PANIdecorated 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. P roton exchange membrane fuel cells (PEMFCs) are regarded as ideal candidates for stationary and mobile power generation because of their high energy conversion efficiencies and low environmental impact.1 However, the insufficient electrocatalytic activity and durability of Pt cathode catalysts still remains a major obstacle for PEMFC applications.2 At present, the most commonly used cathode catalysts are highly dispersed 2−5 nm Pt nanoparticles (NPs) supported on carbon. However, Pt NPs suffer from poor durability because of the rapid and significant loss of platinum electrochemical surface area (ECSA) over time due to corrosion of the carbon support, Pt dissolution, Ostwald ripening, and aggregation. and optimization of the catalyst structure to increase the exposure of Pt NPs to the three-phase zone. 7 Although these methods have been proposed to enhance the catalytic activity and durability, the development of a Pt-based catalyst with both good durability and high mass activity remains a challenge.Conducting polymers such as polypyrrole (PPy) and polyaniline (PANI) have received special attention in fuel cell applications because of their unique π-conjugated structures, which lead to good environmental stability, high electrical and proton conductivity in acidic environments, and unique redox properties. 8 Recently, Deki and co-workers 9 reported the preparation of a Pt/electroconductive-polymer-loaded carbon composite that improved the durability of electrodes in fuel cells. In that study, Pt(NH 3 ) 4 2+ was absorbed onto carbon and used to oxidize aniline while reducing the Pt(NH 3 ) 4 2+ itself. The Pt and PANI were thoroughly mixed together throughout the entire polymerization process. Accordingly, nearly all of the Pt NPs in the prepared Pt/PANI/C composite, except those present on the outermost catalyst layer, were embedded inside the PANI rather than exposed to the outside. Thus, some of the Pt NPs could not be utilized by the fuel cell, and the Pt/PANI/ C composites showed poor oxygen reduction reaction (ORR) activity; indeed, no Pt behavior was observed in cyclic voltammograms (CVs) of ...

Herein, we report a "shape fixing via salt recrystallization" method to efficiently synthesize nitrogen-doped carbon material with a large number of active sites exposed to the three-phase zones, for use as an ORR catalyst. Self-assembled polyaniline with a 3D network structure was fixed and fully sealed inside NaCl via recrystallization of NaCl solution. During pyrolysis, the NaCl crystal functions as a fully sealed nanoreactor, which facilitates nitrogen incorporation and graphitization. The gasification in such a closed nanoreactor creates a large number of pores in the resultant samples. The 3D network structure, which is conducive to mass transport and high utilization of active sites, was found to have been accurately transferred to the final N-doped carbon materials, after dissolution of the NaCl. Use of the invented cathode catalyst in a proton exchange membrane fuel cell produces a peak power of 600 mW cm(-2), making this among the best nonprecious metal catalysts for the ORR reported so far. Furthermore, N-doped carbon materials with a nanotube or nanoshell morphology can be realized by the invented method.

Oxygen vacancies (OVs) are important for changing the geometric and electronic structures as well as the chemical properties of MnO 2 . In this study, we performed a DFT+U calculation on the electronic structure and catalytic performance of a β-MnO 2 catalyst for oxygen reduction reaction (ORR) with different numbers and extents of OVs. Comparing to the experimental XRD analysis, we determined that OVs produce a new crystalline phase of β-MnO 2 . Changes in the electronic structure (Bader charges, band structure, partial density of states (PDOS), local density of states (LDOS), and frontier molecular orbital), proton insertion and oxygen adsorption in β-MnO 2 (110) were investigated as a function of the bulk OVs. The results show that a moderate concentration of bulk OVs reduced the band gap, increased the Fermi and HOMO levels of the MnO 2 (or MnOOH), and elongated the O-O bond of the adsorbed O 2 and co-adsorbed O 2 with H. These changes substantially increase the conductivity of MnO 2 for the catalysis of ORR. However, an excessively high concentration of OVs in β-MnO 2 (110) will work against the catalytic enhancement of MnO 2 for ORR. The DFT+U calculation reveals that a moderate concentration of OVs induced a large overlap of the surface Mn d z2 orbitals and thus introducing an extra donor level at the bottom of the conductive band (CB), which increased the conductivity of β-MnO 2 (110). Such a curvilinear change of the catalytic activity and electronic structure as a function of the oxygen vacancy concentration suggests that the β-MnO 2 with moderate concentration OVs exhibits the highest conductivity and catalytic activity for ORR.

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