RESUMEN
Lithium-sulfur (Li-S) batteries are widely studied because of their high theoretical specific capacity and environmental friendliness. However, the further development of Li-S batteries is hindered by the shuttle effect of lithium polysulfides (LiPSs) and the sluggish redox kinetics. Since the adsorption and catalytic conversion of LiPSs mainly occur on the surface of the electrocatalyst, regulating the surface structure of electrocatalysts is an advisable strategy to solve the obstacles in Li-S batteries. Herein, CoP nanoparticles with high oxygen content on surface embedded in hollow carbon nanocages (C/O-CoP) is employed to functionalize the separators and the effect of the surface oxygen content of CoP on the electrochemical performance is systematically explored. Increasing the oxygen content on CoP surface can enhance the chemical adsorption to lithium polysulfides and accelerate the redox conversions kinetics of polysulfides. The cell with C/O-CoP modified separator can achieve the capacity of 1033 mAh g-1 and maintain 749 mAh g-1 after 200 cycles at 2 C. Moreover, DFT calculations are used to reveal the enhancement mechanism of oxygen content on surface of CoP in Li-S chemistry. This work offers a new insight into developing high-performance Li-S batteries from the perspective of surface engineering.
RESUMEN
The low-cost transition metal oxides have drawn widespread interest as alternatives to noble metal-based electrocatalysts for oxygen evolution reaction (OER). Transition metal oxides usually undergo surface reconstruction during electrochemical reaction to form the actual active species. However, in-depth understanding and regulating of the surface reconstruction of active phases for oxides in OER remains an onerous challenge. Herein, we report a simple Fe element substitution strategy to facilitate the surface reconstruction of spinel oxide NiCr2O4 to generate active (oxy)hydroxides. The activated Fe-doped NiCr2O4 (Act-Fe-NCO) exhibits a lower OER overpotential of 259 mV at 10 mA cm-2 than activated NiCr2O4 (Act-NCO, 428 mV), and shows excellent stability for 120 h. The electrochemically activated CV measurement and nanostructure characterizations reveal that Fe substitution could promote the consumption of lattice oxygen during electrochemical activation to induce the leaching of soluble Cr cations, thereby facilitating the reconstruction of remaining Ni cations on the surface into (oxy)hydroxide active species. Moreover, theoretical calculations further demonstrate that the O 2p band center of NiCr2O4 moves towards the Fermi level due to Fe substitution, thus promoting lattice oxygen oxidation and providing greater structural flexibility for surface reconstruction. This work shows a promising way to regulate the surface reconstruction kinetics and OER electrocatalytic activity of transition metal oxides.
RESUMEN
Transition metal oxides have been extensively investigated for oxygen evolution reaction (OER). While the introduction of oxygen vacancies (Vo) was found to be an effective way to enhance the electrical conductivity and the OER electrocatalytic activity of transition metal oxides, the oxygen vacancies are easily damaged during the long-term catalytic process, resulting in rapid decay of the electrocatalytic activity. Herein, we proposed the strategy of dual-defect engineering to enhance the catalytic activity and stability of NiFe2O4 by filling the oxygen vacancies of NiFe2O4 with phosphorus atoms. The filled P atoms could form coordination with iron and nickel ions to compensate the coordination number and optimize the local electronic structure, which not only enhances the electrical conductivity but also improves the intrinsic activity of the electrocatalyst. Meanwhile, the filling of P atoms could stabilize the Vo and thus improving the cycling stability of the material. The theoretical calculation further demonstrates that the improvement in conductivity and intermediate binding by P refilling remarkably contributes to enhancing the OER activity of NiFe2O4-Vo-P. Benefiting from the synergistic effect of filled P atoms and Vo, the derived NiFe2O4-Vo-P exhibits fascinating activity with ultra-low OER overpotentials of 234 and 306 mV at 10 and 200 mA cm-2, together with the good durability for 120 h at relatively high current density of 100 mA cm-2. This work sheds light on the design of high-performance transition metal oxide catalysts through defect regulation in the future.
RESUMEN
The application of lithium-sulfur (Li-S) batteries is severely hampered by the shuttle effect and sluggish redox kinetics. Herein, amorphous cobalt phosphide grown on a reduced graphene oxide-multiwalled carbon nanotube (rGO-CNT-CoP(A)) is designed as the sulfur host to conquer the above bottlenecks. The differences between amorphous cobalt phosphide (CoP) and crystalline CoP on the surface adsorption as well as conversion of lithium polysulfides (LiPSs) are investigated by systematical experiments and density-functional theory (DFT) calculations. Specifically, the amorphous CoP not only strengthens the chemical adsorption to LiPSs but also greatly accelerates liquid-phase conversions of LiPSs as well as the nucleation and growth of Li2S. DFT calculation reveals that the amorphous CoP possesses higher binding energies and lower diffusion energy barriers for LiPSs. In addition, the amorphous CoP features reduced energy gap and the increased electronic concentrations of adsorbed LiPSs near Fermi level. These characteristics contribute to the enhanced chemisorption ability and the accelerated redox kinetics. Simultaneously, the prepared S/rGO-CNT-CoP(A) electrode delivers an impressive initial capacity of 872 mAh g-1 at 2 C and 617 mAh g-1 can be obtained after 200 cycles, exhibiting excellent cycling stability. Especially, it achieves outstanding electrochemical performance even under high sulfur loading (5.3 mg cm-2) and lean electrolyte (E/S = 7 µLE mg-1S) conditions. This work exploits the application potential for amorphous materials and contributes to the development of highly efficient Li-S batteries.