Modulation of Single Atomic Co and Fe Sites on Hollow Carbon Nanospheres as Oxygen Electrodes for Rechargeable Zn-Air Batteries.

Efficient bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are required for metal air batteries, to replace costly metals, such as Pt and Ir/Ru based compounds, which are typically used as benchmarks for ORR and OER, respectively. Isolated single atomic sites coordinated with nitrogen on carbon supports (M-N-C) have promising performance for replacement of precious metal catalysts. However, most of monometallic M-N-C catalysts demonstrate unsatisfactory bifunctional performance. Herein, a facile way of preparing bimetallic Fe and Co sites entrapped in nitrogen-doped hollow carbon nanospheres (Fe,Co-SA/CS) is explored, drawing on the unique structure and pore characteristics of Zeolitic imidazole frameworks and molecular size of Ferrocene, an Fe containing species. Fe,Co-SA/CS showed an ORR onset potential and half wave potential of 0.96 and 0.86 V, respectively. For OER, (Fe,Co)-SA/CS attained its anodic current density of 10 mA cm-2 at an overpotential of 360 mV. Interestingly, the oxygen electrode activity (ΔE) for (Fe,Co)-SA/CS and commercial Pt/C-RuO2 is calculated to be 0.73 V, exhibiting the bifunctional catalytic activity of (Fe,Co)-SA/CS. (Fe,Co)-SA/CS evidenced desirable specific capacity and cyclic stability than Pt/C-RuO2 mixture when utilized as an air cathode in a homemade Zinc-air battery.

Facile Synthesis of Amorphous Ternary Metal Borides-Reduced Graphene Oxide Hybrid with Superior Oxygen Evolution Activity.

Metal borides represent an emerging family of advanced electrocatalyst for oxygen evolution reaction (OER). Herein, we present a fast and simple method of synthesizing iron-doped amorphous nickel boride on reduced graphene oxide (rGO) sheets. The hybrid exhibits outstanding OER performance and stability in prolonged OER operation. In 1.0 M KOH, only 230 mV is required to afford a current density of 15 mA cm-2 with a small Tafel slope of 50 mV dec-1. DFT calculations lead to a suggestion that the in situ formation of MO xH y during electrochemical activation acts as active sites for water oxidation. The superior OER activity of the as-prepared catalyst is attributed to (i) its unique amorphous structure to allow abundant active sites, (ii) synergistic effect of constituents, and (iii) strong coupling of active material and highly conductive rGO. This work not only provides new perspectives to design a highly effective material for OER but also opens a promising avenue to tailor the electrochemical properties of metal borides, which could be extended to other materials for energy storage and conversion technologies.

Ultrathin Amorphous Iron-Nickel Boride Nanosheets for Highly Efficient Electrocatalytic Oxygen Production.

A cost-effective and efficient electrocatalyst for the oxygen evolution reaction during the electrolysis of water is highly desired. In an effort to develop an economical material for replacing precious-metal-based catalysts, a novel and self-standing amorphous ultrathin nanosheet (NS) of bimetallic iron-nickel boride (Fe-Ni-B NSs) on Ni foam is presented, which displays a better oxygen-evolving activity compared to the precious-metal catalyst RuO2 . In 1.0 m KOH electrolyte solution, it requires an overpotential of only 237 mV to reach a current density of 10 mA cm-2 with a small Tafel slope of 38 mV dec-1 and shows prominent long-term electrochemical stability. A synergistic effect between highly abundant catalytically active sites on the 3D porous substrate improved the electron transport arising from the presence of highly negative boron, and the high conductivity of the substrate results in an outstanding electrocatalytic activity. The advanced catalytic activity, facile electrode fabrication, and low costs make it a potential oxygen-evolving material, which may be extended to other energy-conversion and storage technologies.

Lithiation/Delithiation Synthesis of Few Layer Silicene Nanosheets for Rechargeable Li-O2 Batteries.

Silicene has recently received increasing interest due to its unique properties. However, the synthesis of silicene remains challenging, which limits its wide applications. In this work, a top-down lithiation and delithiation process is developed to prepare few layer silicene-like nanosheets from ball-milled silicon nanopowders. It is found that delithiation solvent plays a critical role in the structure evolution of the final products. The use of isopropyl alcohol renders 2D silicene-like products 30-100 nm in length and ≈2.4 nm in thickness. The electrochemical characterization analysis suggests that the product shows high performance for rechargeable Li-O2 batteries with 73% energy efficiency and high stability. The top-down synthesis strategy proposed in this work not only provides a new solution to the challenging preparation issue of few layer silicene but also demonstrates the feasibility of producing 2D materials from nonlayered starting structures.