The facet-dependent performance has aroused great interest in the fields of catalyst, lithium ion battery and electrochemical sensor. In this study, the well-defined Co 3 O 4 cubes with exposed (001) plane and octahedrons with exposed (111) plane have been successfully synthesized and the facet-dependent electrocatalytic performance of Co 3 O 4 for rechargeable Li−O 2 battery has been comprehensively investigated by the combination of experiments and theoretical calculations. The Li−O 2 battery cathode catalyzed by Co 3 O 4 octahedron with exposed (111) plane shows much higher specific capacity, cycling performance, and rate capability than Co 3 O 4 cube with exposed (001) plane, which may be largely attributed to the richer Co 2+ and more active sites on (111) plane of Co 3 O 4 octahedrons. The DFT-based first-principles calculations further indicate that Co 3 O 4 (111) has a lower activation barrier of O 2 desorption in oxygen evolution reaction (OER) than Co 3 O 4 (001), which is very important to refresh active sites of catalyst and generate a better cyclic performance. Also, our calculations indicate that Co 3 O 4 (111) surface has a stronger absorption ability for Li 2 O 2 than Co 3 O 4 (001), which may be an explanation for a larger initial capacity in Co 3 O 4 (111) plane by experimental observation.

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To solve the stability issue of cost-effective nonfluorinated membranes, an ether-free poly(arylene piperidinium) (PBPip)-based membrane is first applied in redox flow batteries (RFBs). For improved efficiencies of RFB, amphoteric side chains are introduced onto the PBPip. Without an ether bond in the polymer backbone, the membrane shows a good stability in a strong oxidation environment. The Fourier transform infrared (FTIR) spectra exhibit no obvious changes over 30 days of oxidation test. Different from traditional blended amphoteric membranes, the amphoteric side chain allows both cation-and anion-exchange capacities to increase with grafting degree, which leads to a very high total ion-exchange capacity (IEC) (4.19 mmol g −1 ). Outstanding ion-conduction ability (area resistance: 0.22 Ω cm 2 ) comparable to Nafion 212 (0.24 Ω cm 2 ) is consequently achieved. Ionic cross-linking structure between cationic and anionic groups results in a low swelling rate (13.9%). Combined with the repelling effect of positively charged piperidinium, a low VO 2+ permeability (1.31 × 10 −8 cm 2 s −1 ) is accomplished. On the basis of these good properties, the membrane exhibits excellent vanadium battery performances, especially at high current densities. The VE and EE both exceed 80% even at 200 mA cm −2 . The battery performances have no obvious reductions after 500 cycles. These results indicate that this work provides a new orientation to design the membrane for RFB.

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