Jing Du

Ultrasmall Sn Nanoparticles Embedded in Nitrogen-Doped Porous Carbon As High-Performance Anode for Lithium-Ion Batteries

In this Letter, we reported on the preparation and Li-ion battery anode application of ultrasmall Sn nanoparticles (∼5 nm) embedded in nitrogen-doped porous carbon network (denoted as 5-Sn/C). Pyrolysis of Sn(Salen) at 650 °C under Ar atmosphere was carried out to prepare N-doped porous 5-Sn/C with the BET specific surface area of 286.3 m(2) g(-1). The 5-Sn/C showed an initial discharge capacity of 1014 mAh g(-1) and a capacity retention of 722 mAh g(-1) after 200 cycles at the current density of 0.2 A g(-1). Furthermore, a reversible capacity of ∼480 mAh g(-1) was obtained at much higher current density of 5 A g(-1). The remarkable electrochemical performance of 5-Sn/C was attributed to the effective combination of ultrasmall Sn nanoparticles, uniform distribution, and porous carbon network structure, which simultaneously solved the major problems of pulverization, loss of electrical contact, and particle aggregation facing Sn anode.

Enhancing Electrocatalytic Oxygen Reduction on MnO2 with Vacancies

Oxygen-vacant nanocrystalline MnO(2) has been prepared by the simple process of annealing pristine oxide in Ar or O(2) . Both experimental and computational studies indicate that the catalytic activity of MnO(2) towards oxygen reduction is enhanced by introducing a modest concentration of oxygen vacancies.

Porous calcium–manganese oxide microspheres for electrocatalytic oxygen reduction with high activity

A series of calcium–manganese oxides (Ca–Mn–O) were prepared through thermal decomposition of carbonate solid–solution precursors and investigated as electrocatalysts for oxygen reduction reaction (ORR). The synthesized crystalline Ca–Mn–O compounds, including perovskite-type CaMnO3, layered structured Ca2Mn3O8, post-spinel CaMn2O4 and CaMn3O6, presented similar morphologies of porous microspheres with agglomerated nanoparticles. Electrochemical results, surface analysis, and computational studies revealed that the catalytic activities of Ca–Mn–O oxides, in terms of onset potential, reduction current, and transferred electron number, depended strongly on both the surface Mn oxidation state and the crystallographic structures. Remarkably, the as-synthesized CaMnO3 and CaMn3O6 exhibited considerable activity and enabled an apparent quasi 4-electron oxygen reduction with low yield of peroxide species in alkaline solutions, suggesting their potential applications as cheap and abundant ORR catalysts.

Nonstoichiometric Perovskite CaMnO3−δ for Oxygen Electrocatalysis with High Activity

Perovskite oxides offer efficient and cheap electrocatalysts for both oxygen reduction reactions and oxygen evolution reactions (ORR/OER) in diverse oxygen-based electrochemical technologies. In this study, we report a facile strategy to enhance the electrocatalytic activity of CaMnO3 by introducing oxygen defects. The nonstoichiometric CaMnO(3-δ) (0 < δ ≤ 0.5) was prepared through thermal reduction of pristine perovskite microspheres and nanoparticles, which were synthesized from thermal-decomposition of carbonate precursors and the Pechini route, respectively. The as-prepared samples were analyzed by chemical titration, structural refinement, thermogravimetric analysis, and energy spectrometry. In 0.1 M KOH aqueous solution, the nonstoichiometric CaMnO(3-δ) with δ near 0.25 and an average Mn valence close to 3.5 exhibited the highest ORR activity (36.7 A g(-1) at 0.70 V vs RHE, with onset potential of 0.96 V), which is comparable to that of benchmark Pt/C. Density functional theory (DFT) studies and electrical conductivity measurement revealed that the enhanced ORR kinetics is due to facilitated oxygen activation and improved electrical properties. Besides high activity, the nonstoichiometric perovskite oxides showed respectable catalytic stability. Furthermore, the moderate oxygen-defective CaMnO(3-δ) (δ ≈ 0.25) favored the OER because of the improved electrical conductivity. This study makes nonstoichiometric CaMnO(3-δ) a promising active, inexpensive bifunctional catalytic material for reversible ORR and OER.

M(Salen)-derived Nitrogen-doped M/C (M = Fe, Co, Ni) Porous Nanocomposites for Electrocatalytic Oxygen Reduction

Carbonaceous materials containing non-precious metal and/or doped nitrogen have attracted tremendous attention in the field of electrochemical energy storage and conversion. Herein, we report the synthesis and electrochemical properties of a new family of nitrogen-doped metal/carbon (M/N/C, M = Fe, Co, Ni) nanocomposites. The M/N/C nanocomposites, in which metal nanoparticles are embedded in the highly porous nitrogen-doped carbon matrix, have been synthesized by simply pyrolyzing M(salen) (salen = N,N′-bis(salicylidene)-ethylenediamine) complex precursors. The prepared Co/N/C and Fe/N/C exhibit remarkable electrocatalytic activity (with onset potential of 0.96 V for Fe/N/C and half-wave potential of 0.80 V for Co/N/C) and high stability for the oxygen reduction reaction (ORR). The superior performance of the nanocomposites is attributed to their bimodal-pore structure, high surface area, as well as uniform distribution of high-density nitrogen and metal active sites.

Facile solvothermal synthesis of CaMn2O4 nanorods for electrochemical oxygen reduction

Electrocatalysts for the oxygen reduction reaction (ORR) are of pivotal importance in various fuel cells and metal–air batteries. In this study, we report a facile synthesis of one-dimensional (1D) CaMn2O4 nanostructures and their applications as cheap and active electrocatalysts for the ORR. Marokite CaMn2O4 nanorods with post-spinel phase were prepared by a solvothermal route at mild temperatures, using potassium manganese oxide hydrate and calcium nitrate as the precursors and ethanol as the solvent. The as-prepared nanorods adopted the orthorhombic structure and possessed diameters of 150–300 nm and lengths of 2–4 μm, with preferentially exposed (023) planes on surfaces. In alkaline electrolytes, CaMn2O4 nanorods exhibited considerable catalytic performance and enabled an apparent quasi-four-electron transfer in the ORR, as evidenced by rotating disk electrode and rotating ring-disk electrode studies. The determined Tafel slop and the chronoamperometry stability of CaMn2O4 nanorod electrocatalysts were comparable to the counterpart Pt nanoparticles supported on carbon.

Rapid Synthesis and Efficient Electrocatalytic Oxygen Reduction/Evolution Reaction of CoMn2O4 Nanodots Supported on Graphene

Transition-metal oxides have attracted extensive interest as oxygen-reduction/evolution reaction (ORR/OER) catalyst alternatives to precious Pt-based materials but generally exhibit limited electrocatalytic performance due to their large overpotential and low specific activity. We here report a rapid synthesis of spinel-type CoMn2O4 nanodots (NDs, below 3 nm) monodispersed on graphene for highly efficient electrocatalytic ORR/OER in 0.1 M KOH solution. The preparation of the composite involves the reaction of manganese and cobalt salts in mixed surfactant-solvent-water solution at mild temperature (120 °C) and air. CoMn2O4 NDs homogeneously distributed on carbonaceous substrates show strong coupling and facile charge transfer. Remarkably, graphene-supported CoMn2O4 NDs showed 20 mV higher ORR half-wave potential, twice the kinetic current, and better catalytic durability compared to the benchmark carbon-supported Pt nanoparticles (Pt/C). Moreover, CoMn2O4/reduced graphene oxide afforded electrocatalytic OER with a current density of 10 mA cm(-2) at a low potential of 1.54 V and a small Tafel slope of ∼56 mV/dec. This indicates that the composite of CoMn2O4 nanodots monodispersed on graphene is promising as highly efficient bifunctional electrocatalysts of ORR and OER that can be used in the areas of fuel cells and rechargeable metal-air batteries.

Porous perovskite calcium–manganese oxide microspheres as an efficient catalyst for rechargeable sodium–oxygen batteries

We report herein the preparation of porous CaMnO3 microspheres and their electrochemical catalytic performance as a cathode for rechargeable sodium–oxygen (Na–O2) batteries. In ether-based electrolytes, the CaMnO3/C cathode exhibits a high discharge capacity of 9560 mA h g−1 at a current density of 100 mA g−1, high rate capability (a capacity of 1940 mA h g−1 at 1000 mA g−1), and considerable cyclability up to 80 cycles. Two discharged species of NaO2 and Na2O2 are detected at the discharged state. The remarkable electrocatalytic activity of CaMnO3 both for the oxygen reduction reaction (ORR) and for the oxygen evolution reaction (OER) is attributed to the porous micro–nanostructures in stable ether-based electrolytes.

The anion effect on the oxygen reduction of MnX (X = O, S, and Se) catalysts

In this study, a series of Mn-based compounds, MnX (X = O, S, and Se), were prepared and investigated as electrocatalysts for the oxygen reduction reaction (ORR). These compounds adopt the same rock-salt-type crystal structure and present a similar morphology and close particle size. Among them, the unreported MnSe catalyst exhibits the best performance towards the ORR, in terms of onset potential, kinetic current density, and four-electron selectivity, as demonstrated by cyclic voltammetry (CV) and polarization measurements. The different activities of MnX can be ascribed to the anion, which can assist in the change of the Mn valance state. Besides, the methanol tolerance and stability of MnSe are comparable to those of commercial Pt/C. Our results would enlighten the rational design of transition metal-based ORR catalytic materials by taking advantages of the anion effect.

Hierarchical porous Fe3O4/Co3S4 nanosheets as an efficient electrocatalyst for the oxygen evolution reaction

Transition-metal sulfides have attracted extensive interest as oxygen evolution reaction (OER) catalyst alternatives to noble metal based catalysts but generally exhibit limited electrocatalytic activity. We here report hierarchical porous Fe3O4/Co3S4 composite nanosheets for a highly efficient electrocatalytic OER in alkaline electrolytes. The preparation of the composite involves a hydrothermal synthesis and a subsequent sulfurization process. The prepared porous Fe3O4/Co3S4 composite shows strong synergetic coupling effects, facilitating active site exposure, and facile charge transfer. Remarkably, Fe3O4/Co3S4 nanosheets afforded an electrocatalytic OER with a current density of 10 mA cm−2 at a low potential of 1.50 V and a small Tafel slope of about 56 mV dec−1. This indicates that the composite of porous Fe3O4/Co3S4 nanosheets is promising as an efficient electrocatalyst for the OER that can be used in the fields of fuel cells, metal-air batteries and water splitting for hydrogen production.