The long-standing issues of low intrinsic electronic conductivity, slow lithium-ion diffusion and irreversible phase transitions on deep discharge prevent the high specific capacity/energy (443 mAh g À 1 and 1,550 Wh kg À 1 ) vanadium pentoxide from being used as the cathode material in practical battery applications. Here we develop a method to incorporate graphene sheets into vanadium pentoxide nanoribbons via the sol-gel process. The resulting graphene-modified nanostructured vanadium pentoxide hybrids contain only 2 wt. % graphene, yet exhibits extraordinary electrochemical performance: a specific capacity of 438 mAh g À 1 , approaching the theoretical value (443 mAh g À 1 ), a long cyclability and significantly enhanced rate capability. Such performance is the result of the combined effects of the graphene on structural stability, electronic conduction, vanadium redox reaction and lithium-ion diffusion supported by various experimental studies. This method provides a new avenue to create nanostructured metal oxide/graphene materials for advanced battery applications.

The enhancement of Pt nanoparticle anchoring strength and dispersion on carbon supports is highly desirable in polymer electrolyte membrane fuel cells (PEMFCs) as well as in other catalysis processes. Presented here is a comprehensive study of the interaction between catalyst nanoparticles and carbon supports in terms of the electronic structure change and its effects on the electrocatalytic performance of supported catalysts. Graphene was chosen as an ideal model support because the unique 2-D structure allows the direct investigation of the interaction with supported metal nanoparticles at their interface. We developed a facile strategy to covalently graft p-phenyl SO3Hor p-phenyl NH2groups onto the graphene surface. The functional groups were found to not only facilitate the homogeneous distribution of Pt nanoparticles on the surface of graphene supports and reduce the Pt average particle size but also strengthen the interaction of the Pt atoms with the functional groups and, consequently, minimize the migration/coalescence of the Pt nanoparticles in the course of accelerated durability tests. The experimental results from both X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) demonstrate the electron density shift from Pt to graphene supports with the strength of the Pt–graphene interaction following the trend of Pt/p-phenyl NH2-graphene > Pt/p-phenyl SO3H-graphene > Pt/graphene. This study will shed light on strategies to improve not only the durability but also the activity of the metal nanoparticles via the functionalization of the catalyst supports in the catalysis field.

Graphene/polyaniline (PANI) nanocomposites were prepared by reducing graphene oxide with hydrazine in the presence of different amounts of polyaniline nanoparticles. In situ cryo-transmission electron microscope (TEM) images of a graphene oxide (GO)/PANI solution revealed that the PANI nanoparticles were anchored on the surface of the GO sheets. During the reduction, the as-adsorbed PANI nanoparticles were sandwiched between layers of graphene sheets. These PANI nanoparticles acted as spacers to create gaps between neighboring graphene sheets, resulting in a higher surface area compared to pure graphene. Graphene/PANI nanocomposites exhibited the high specific surface area of 891 m2/g. Utilizing this composite material, a supercapacitor with a specific capacitance of 257 F/g at a current density of 0.1 A/g has been achieved.

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The failure mechanism of LiFePO4 cells in over-discharge conditions has been systematically studied using commercial A123 18650 cells at a 1.0 C rate and different conditions – from 5% to 20% over-discharge (DOD = 105% to 120%). SEM/EDAX, high-energy synchrotron XRD (HESXRD), and cyclic voltammetry (CV) were used to characterize the morphology, structure, and electrode potentials of cell components both in situ and ex situ. The failure behaviors of A123 18650 cells experiencing different degrees of over-discharge were found to be similar, and the 20% over-discharge process was analyzed as the representative example. The Cu electrochemical potentials in the 1.2 M LiPF6 EC/EMC electrolyte were measured during the charge/over-discharge process using CV, proving that Cu oxidation and reduction in the cell during the charge/over-discharge cycle were theoretically possible to proceed. A possible failure mechanism is proposed: during the over-discharging process, Cu foil oxidized first to Cu+, then to Cu2+ cations; next, these Cu+ and Cu2+ cations diffused to the cathode side from the anode side; and finally, these Cu2+ cations reduced to Cu+ cations, and then reduced further, back to metallic Cu. During charge/over-discharge cycling, Cu dendrites continued growing from the cathode side, penetrating through the separator and forming a copper bridge between the anode and cathode. The copper bridge caused micro-shorting and eventually led to the failure of the cell. During the charge/over-discharge cycles, the continued cell temperature increase at the end of over-discharge is evidence of the micro-shorting.

The ammonia electro-oxidation reaction (AOR) has been studied due to its promising applications in ammonia electrolysis, wastewater remediation, direct ammonia fuel cells, and sensors. However, it is difficult to compare and analyze the reported electrocatalytic activity of AOR reliably, likely due to the variation in catalyst synthesis, electrode composition, electrode morphology, and testing protocol. In this paper, the electro-oxidation of ammonia on different carbon-supported precious metal nanoparticle catalysts was revisited. The effect of experimental conditions, electrochemical test parameters, electrocatalytic activity, thermodynamics, and possible deactivation mechanism of the catalysts were investigated. Pt/C catalyst possesses the highest electrocatalytic activity, while Ir/C and Rh/C show lower overpotential. The onset potential of the AOR is related to the hydrogen binding energy of the catalyst. N ads is one major cause of deactivation accompanied with the formation of surface O/OH ads at high potentials. The coulombic efficiency of N ads formation on Pt is about 1% initially and gradually decreases with reaction time. Increase in ammonia concentration leads to increase in current density, while increase in hydroxyl ions concentration can enhance the current density and reduce the overpotential simultaneously. The slopes of AOR onset potential and hydrogen adsorption/desorption potential of Pt/C as a function of pH follow Nernst equation. In contrast, potentials measured at different current densities exhibit non-Nernstian behavior, suggesting a critical role of the local pH change.

A simple method to fabricate graphene-encapsulated pyrolyzed polyaniline-grafted Si nanoparticles has been developed. Instead of using Si nanoparticles with a native oxide layer, HF-treated Si nanoparticles were employed in this work. The uniqueness of this method is that, first, a PANI layer over the Si nanoparticles was formed via the surface-initiated polymerization of aniline on the surface of aniline-functionalized Si nanoparticles; then, the PANI-grafted Si nanoparticles were wrapped by the GO sheets via π-π interaction and electrostatic attraction between the GO and the PANI. Finally, the GO and PANI were pyrolyzed, and this pyrolyzed PANI layer tightly binds the graphene sheets and the Si nanoparticles together in the composite. The composite materials exhibit better cycling stability and Coulombic efficiency as anodes in lithium ion batteries, as compared to pure Si nanoparticles and physically mixed graphene/Si composites. After 300 cycles at a current density of 2 A/g, the composite electrodes can still deliver a specific capacity of about 900 mAh/g, which corresponds to ∼76% capacity retention. The enhanced performance can be attributed to the absence of surface oxides, the better electronic conductivity, faster ion diffusion rate, and the strong interaction between the graphene sheets and the tightly bound carbon-coated Si nanoparticles.

We have performed operando synchrotron high-energy X-ray diffraction (XRD) to obtain nonintrusive, real-time monitoring of the dynamic chemical and structural changes in commercial 18650 LiFePO4/C cells under realistic cycling conditions. The results indicate a nonequilibrium lithium insertion and extraction in the LiFePO4 cathode, with neither the LiFePO4 phase nor the FePO4 phase maintaining a static composition during lithium insertion/extraction. On the basis of our observations, we propose that the LiFePO4 cathode simultaneously experiences both a two-phase reaction mechanism and a dual-phase solid-solution reaction mechanism over the entire range of the flat voltage plateau, with this dual-phase solid-solution behavior being strongly dependent on charge/discharge rates. The proposed dual-phase solid-solution mechanism may explain the remarkable rate capability of LiFePO4 in commercial cells.