are still severe technological challenges in the wide practical applications for LOBs, such as poor round-trip efficiency, high overpotential, inferior cycle stability, and terrible rate capability. The major factor constraining the performance advantage of LOBs is the sluggish kinetics of the oxygen reduction reaction (ORR, discharge process) and the oxygen evolution reaction (OER, charge process) on the cathode. In the discharge process, the surface of cathode is gradually shrouded by Li 2 O 2 as the insulative discharging product, which reduces the conductivity of electrode system and increased the decomposed energy barrier of surface products, eventually resulting in excessive overpotential and inferior cycle performance. Furthermore, the generated superoxide radicals would attack the electrolyte and the cathode materials in the discharge/ charge process, causing the side reactions and formation of various by-products on the electrode. The development of highefficient cathode catalyst is one of the most important strategies to solve the problems of LOBs.Carbon materials were first studied as cathode materials because of the low cost and good conductivity characteristics. However, due to the superoxide radicals generated during the ORR/OER process, a large amount of side products (Li 2 CO 3 ) is produced, which contributes to the poor cycle stability. [3] Researchers have been working on the development of novel catalysts with high efficiency and their electrochemical reaction mechanisms to reduce by-products and improve cycle life and efficiency, such as precious metals (Pt, [4][5][6][7] Au, [8] Pd, [4] and Ru [9,10] ), metal oxides (CeO 2 , [11] RuO 2 , [12] NiFeO, [13] NiCo 2 O 4 , [14] etc.), metal carbides/nitrides (TiC, [15] Mo 2 C, [16] CoN, [17] etc.). High-efficient cathode catalysts could dominate the nucleation and growth kinetics, morphology, crystal states, and chemical composition of discharge products. During the ORR/OER process, LiO 2 is considered to be an important intermediate, which greatly affects the stability of catalyst and composition of discharge products. [18][19][20][21][22][23][24] The formation of discharge products could proceed through surface or solution pathway with LiO 2 species at the interface of cathode and electrolyte. Moreover, the morphology of the discharge products formed under the assistant of the cathode catalysts is strongly related to the cycle stability. Toroid like discharge products consisting of Li 2−x O 2 and Li 2 O 2 could be obtained on carbon-based material Highly-efficient cathode catalysts are the key to improve high rate cycle stability, avoid side reactions, and lower the overpotential of lithium-oxygen batteries (LOBs). MXenes are predicted to be one of the most impressive materials for energy applications. In this work, the catalytic capability of Nb 2 C MXene is demonstrated with a uniform O-terminated surface as a cathode material for LOBs. The easily fabricated uniform O-terminated surface, high catalytic activity of Nb 2 CO 2 sites, and unique re...
In this paper, the electronic structures of NbO2 and Nb2O5 are theoretically and experimentally analyzed. The oxides in the samples are mainly consisted of NbO2 and NbO, whereas the outmost layer of the samples is NbO2. After exposure to air, the outermost layer on all niobium samples is Nb2O5. The photoelectrons from the first 2–4 Å contribute to the spectra, so the valence band structure of NbO2 and Nb2O5 can be confirmed from ultraviolet photoelectron spectroscopy (UPS). By comparing the UPS with density of state results, the electronic structure of NbO2 and Nb2O5 can be distinguished from each other, and then the electronic structure was deconvoluted into several electronic states. The agreement between experimental result and theory is, in the best case, satisfactory. Copyright © 2013 John Wiley & Sons, Ltd.
The electrocatalytic reduction conversion of CO2 to produce methane (CH4) as a fuel has attracted intensive attention for renewable energy.
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