Spinel Nickel Cobaltite Mesostructures Assembled from Ultrathin Nanosheets for High-Performance Electrochemical Energy Storage

Transition metal oxides (TMOs) are promising electrode materials for advanced electrochemical energy storage (EES) due to their high theoretical capacities, but they usually exhibit quite poor practical performance. There is a pressing need to boost their EES performance by electrode engineering directed with a well-defined structure–performance relationship. Herein, we report an efficient approach to improve the specific capacitance and high-rate capability of spinel nickel cobaltite by constructing three-dimensional (3D) hierarchical porous mesostructures. The optimal Ni_1.4Co_1.6O₄ mesostructures assembled from ultrathin nanosheets exhibit high capacitance (2282 F g^–1 at 1 A g^–1), excellent high-rate capability (1234 F g^–1 at 50 A g^–1) and good cycling performance, which are significantly superior to the Co₃O₄ mesostructure counterparts, Ni_1.4Co_1.6O₄ mesostructures assembled from nanowires, and randomly packed Ni_1.4Co_1.6O₄ nanosheets. The excellent performance is attributed to the stable hierarchical porous architecture which enables a large electroactive area and synergistically enhanced electrolyte access, solid-state ion diffusion, and electron transfer. This tactic of constructing a 3D mesostructured electrode with enhanced charge transport can be generalized to other TMOs for improving their EES performances

Significant Contribution of Intrinsic Carbon Defects to Oxygen Reduction Activity

While the field of carbon-based metal-free electrocatalysts for oxygen reduction reaction (ORR) has experienced great progress in recent years, the fundamental issue of the origin of ORR activity is far from being clarified. To date, the ORR activities of these electrocatalysts are usually attributed to different dopants, while the contribution of intrinsic carbon defects has been explored little. Herein, we report the high ORR activity of the defective carbon nanocages, which is better than that of the B-doped carbon nanotubes and comparable to that of the N-doped carbon nanostructures. Density functional theory calculations indicate that pentagon and zigzag edge defects are responsible for the high ORR activity. The mutually corroborated experimental and theoretical results reveal the significant contribution of the intrinsic carbon defects to ORR activity, which is crucial for understanding the ORR origin and exploring the advanced carbon-based metal-free electrocatalysts

Monodispersed Ru Nanoparticles Functionalized Graphene Nanosheets as Efficient Cathode Catalysts for O₂‑Assisted Li–CO₂ Battery

In Li–CO₂ battery, due to the highly insulating nature of the discharge product of Li₂CO₃, the battery needs to be charged at a high charge overpotential, leading to severe cathode and electrolyte instability and hence poor battery cycle performance. Developing efficient cathode catalysts to effectively reduce the charge overpotential represents one of key challenges to realize practical Li–CO₂ batteries. Here, we report the use of monodispersed Ru nanoparticles functionalized graphene nanosheets as cathode catalysts in Li–CO₂ battery to significantly lower the charge overpotential for the electrochemical decomposition of Li₂CO₃. In our battery, a low charge voltage of 4.02 V, a high Coulomb efficiency of 89.2%, and a good cycle stability (67 cycles at a 500 mA h/g limited capacity) are achieved. It is also found that O₂ plays an essential role in the discharge process of the rechargeable Li–CO₂ battery. Under the pure CO₂ environment, Li–CO₂ battery exhibits negligible discharge capacity; however, after introducing 2% O₂ (volume ratio) into CO₂, the O₂-assisted Li–CO₂ battery can deliver a high capacity of 4742 mA h/g. Through an in situ quantitative differential electrochemical mass spectrometry investigation, the final discharge product Li₂CO₃ is proposed to form via the reaction 4Li⁺ + 2CO₂ + O₂ + 4e^– → 2Li₂CO₃. Our results validate the essential role of O₂ and can help deepen the understanding of the discharge and charge reaction mechanisms of the Li–CO₂ battery