中央研究院 原子與分子科學研究所

The specific capacity of commercially available cathode carbon coated LiFePO₄ is typically 120-160 mAh g^-1, lower than the theoretical value 170 mAh g^-1. We discover that the carbon coated LiFePO₄ surface-modified with 2 wt% of the electrochemically exfoliated graphene layers is able to reach 208 mAh g^-1 in specific capacity. The excess capacity is attributed to the reversible reduction-oxidation reaction between the lithium ions of the electrolyte and the exfoliated graphene flakes. The highly conductive grapheneflakes wrapping around carbon coated LiFePO₄ also assist the electron migration during the charge/discharge processes, diminishing the irreversible capacity at the first cycle and leading to ~100% Coulombic efficiency.  Such a scalable approach can be applied to other cathode systems, boosting up the capacity for various Li batteries.

A 3D Ni foam deposited with graphene layers on surfaces was used as a conducting solid support to load MoSx catalysts for electrocatalytic hydrogen evolution. The graphene sheets grown on Ni foams provide robust protection and efficiently increase its stability in acid. The hydrogen evolution rate reaches 302 ml g-1cm-2h-1 (13.47 mmol g-1cm-2h-1) at overpotential V=0.2V and the catalytic species were likely related to the S22-. The developments in graphene based 3D electrodes may further advance the efficiency of various electrocatalytic reactions, which warrants more investigations.

The two-dimensional layer of molybdenum disulfide (MoS₂) has recently attracted much interest due to its direct-gap property and potential applications in optoelectronics and energy harvesting. However, the synthetic approach to obtain high-quality and large-area MoS₂ atomic thin layers is still rare. Here we report that the high-temperature annealing of a thermally decomposed ammonium thiomolybdate layer in the presence of sulfur can produce large-area MoS₂ thin layers with superior electrical performance on insulating substrates. Spectroscopic and microscopic results reveal that the synthesized MoS₂ sheets are highly crystalline. The electron mobility of the bottom-gate transistor devices made of the synthesized MoS₂ layer is comparable with those of the micromechanically exfoliated thin sheets from MoS₂ crystals. This synthetic approach is simple, scalable, and applicable to other transition metal dichalcogenides. Meanwhile, the obtained MoS₂ films are transferable to arbitrary substrates, providing great opportunities to make layered composites by stacking various atomically thin layers.

Large-area MoS₂ atomic layers are synthesized on SiO₂ substrates by chemical vapor deposition using MoO₃ and S powders as the reactants. Optical, microscopic and electrical measurements suggest that the synthetic process leads to the growth of MoS₂ monolayer. The TEM images verify that the synthesized MoS₂ sheets are highly crystalline.