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  • Article
    Bai F, Qu X, Wang J, Chen X, Yang W.
    ACS Appl Mater Interfaces. 2020 Jul 29;12(30):33740-33750.
    Oxygen reduction reaction (ORR) is an important cathode reaction in fuel cells and metal-air batteries. Composites of transition-metal sulfides (TMSs) and nitrogen-doped carbon (NC) are promising alternative ORR catalysts because of their high catalytic activity. However, the agglomeration of TMS particles limits practical applications. Here, a confinement catalyst composed of Co9S8@NC with a flower-like morphology was derived from metanilic intercalated Co(OH)2 through interlayer-confined carbonation accompanied with host-layer sulfidation. The surface of the Co9S8 particles is covered with a few layers of nitrogen-doped graphene, which can prevent the Co9S8 particles from agglomeration and also produce catalytic activity affected by internal Co9S8. Thus, the Co9S8@NC material achieves excellent ORR performance with a half-wave potential of 0.861 VRHE. In addition, an oxide layer on the surface of Co9S8@NC is fabricated shortly after the ORR starts. Further tests and density functional theory calculations indicated that this cobalt oxide layer can increase the electrochemically active area of Co9S8@NC as well as reduce the ORR energy barrier, thereby providing more catalytic active sites and enhancing the intrinsic catalytic activity, thus achieving a self-activation effect during the electrochemical reaction process.
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  • Article
    Wei X, Zhang T, Luo T.
    ACS Appl Mater Interfaces. 2017 Oct 04;9(39):33740-33748.
    Thermal transport across hard-soft interfaces is critical to many modern applications, such as composite materials, thermal management in microelectronics, solar-thermal phase transition, and nanoparticle-assisted hyperthermia therapeutics. In this study, we use equilibrium molecular dynamics (EMD) simulations combined with the Green-Kubo method to study how molecularly heterogeneous structures of the self-assembled monolayer (SAM) affect the thermal transport across the interfaces between the SAM-functionalized gold and organic liquids (hexylamine, propylamine and hexane). We focus on a practically synthesizable heterogeneous SAM featuring alternating short and long molecular chains. Such a structure is found to improve the thermal conductance across the hard-soft interface by 46-68% compared to a homogeneous nonpolar SAM. Through a series of further simulations and analyses, it is found that the root reason for this enhancement is the penetration of the liquid molecules into the spaces between the long SAM molecule chains, which increase the effective contact area. Such an effect is similar to the fins used in macroscopic heat exchanger. This "molecular fin" structure from the heterogeneous SAM studied in this work provides a new general route for enhancing thermal transport across hard-soft material interfaces.
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  • Article
    Jabeen N, Xia Q, Savilov SV, Aldoshin SM, Yu Y, Xia H.
    ACS Appl Mater Interfaces. 2016 Dec 14;8(49):33732-33740.
    Although the theoretical capacitance of MnO2 is 1370 F g-1 based on the Mn3+/Mn4+ redox couple, most of the reported capacitances in literature are far below the theoretical value even when the material goes to nanoscale. To understand this discrepancy, in this work, the electrochemical behavior and charge storage mechanism of K+-inserted α-MnO2 (or KxMnO2) nanorod arrays in broad potential windows are investigated. It is found that electrochemical behavior of KxMnO2 is highly dependent on the potential window. During cyclic voltammetry cycling in a broad potential window, K+ ions can be replaced by Na+ ions, which determines the pseudocapacitance of the electrode. The K+ or Na+ ions cannot be fully extracted when the upper cutoff potential is less than 1 V vs Ag/AgCl, which retards the release of full capacitance. As the cyclic voltammetry potential window is extended to 0-1.2 V, enhanced specific capacitance can be obtained with the emerging of new redox peaks. In contrast, the K+-free α-MnO2 nanorod arrays show no redox peaks in the same potential window together with much lower specific capacitance. This work provides new insights on understanding the charge storage mechanism of MnO2 and new strategy to further improve the specific capacitance of MnO2-based electrodes.
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