Rambutan-Like FeCO₃ Hollow Microspheres: Facile Preparation and Superior Lithium Storage Performances

Rambutan-like FeCO₃ hollow microspheres were prepared via a facile and economic one-step hydrothermal method. The structure and morphology evolution mechanism was disclosed through time-dependent experiments. After undergoing the symmetric inside-out Ostwald ripening, the resultants formed microporous/nanoporous constructions composed of numerous one-dimensional (1D) nanofiber building blocks. Tested as anode materials of Li-ion batteries, FeCO₃ hollow microspheres presented attractive electrochemical performances. The capacities were over 1000 mAh g^–1 for initial charge, ∼880 mAh g^–1 after 100 cycles at 50 mA g^–1, and ∼710 mAh g^–1 after 200 cycles at 200 mA g^–1. The 1D nanofiber assembly and hollow interior endow this material efficient contact with electrolyte, short Li⁺ diffusion paths, and sufficient void spaces to accommodate large volume variation. The cost-efficient FeCO₃ with rationally designed nanostructures is a promising anode candidate for Li-ion batteries

Co₂(OH)₂CO₃ Nanosheets and CoO Nanonets with Tailored Pore Sizes as Anodes for Lithium Ion Batteries

Co₂(OH)₂CO₃ nanosheets were prepared and initially tested as anode materials for Li ion batteries. Benefiting from hydroxide and carbonate, the as-prepared sample delivered a high reversible capacity of 800 mAh g^–1 after 200 cycles at 200 mA g^–1 and long-cycling capability of 400 mAh g^–1 even at 1 A g^–1. Annealed in Ar, monoclinic Co₂(OH)₂CO₃ nanosheets were transformed into cubic CoO nanonets with rich pores. The pore size had apparent influence on the high-rate performances of CoO. CoO with appropriate pore sizes exhibited greatly enhanced Li storage performances, stable capacity of 637 mAh g^–1 until 200 cycles at 1 A g^–1. More importantly, after many fast charge–discharge cycles, the highly porous nanonets were still maintained. Our results indicate that Co₂(OH)₂CO₃ nanosheets and highly porous CoO nanonets are both promising candidate anode materials for high-performance Li ion batteries

Mechanistic Insights into Surface Chemical Interactions between Lithium Polysulfides and Transition Metal Oxides

The design and development of materials for electrochemical energy storage and conversion devices requires fundamental understanding of chemical interactions at electrode/electrolyte interfaces. For Li–S batteries that hold the promise for outperforming the current generation of Li ion batteries, the interactions of lithium polysulfide (LPS) intermediates with the electrode surface strongly influence the efficiency and cycle life of the sulfur cathode. While metal oxides have been demonstrated to be useful in trapping LPS, the actual binding modes of LPS on 3d transition metal oxides and their dependence on the metal element identity across the periodic table remain poorly understood. Here, we investigate the chemical interactions between LPS and oxides of Mn, Fe, Co, and Cu by combining X-ray photoelectron spectroscopy and density functional theory calculations. We find that Li–O interactions dominate LPS binding to the oxides (Mn₃O₄, Fe₂O₃, and Co₃O₄), with increasing strength from Mn to Fe to Co. For Co₃O₄, LPS binding also involves metal–sulfur interactions. We also find that the metal oxides exhibit different binding preferences for different LPS, with Co₃O₄ binding shorter-chain LPS more strongly than Mn₃O₄. In contrast to the other oxides, CuO undergoes intense reduction and dissolution reactions upon interaction with LPS. The reported findings are thus particularly relevant to the design of LPS/oxide interfaces for high-performance Li–S batteries

Surface Chemistry in Cobalt Phosphide-Stabilized Lithium–Sulfur Batteries

Chemistry at the cathode/electrolyte interface plays an important role for lithium–sulfur batteries in which stable cycling of the sulfur cathode requires confinement of the lithium polysulfide intermediates and their fast electrochemical conversion on the electrode surface. While many materials have been found to be effective for confining polysulfides, the underlying chemical interactions remain poorly understood. We report a new and general lithium polysulfide-binding mechanism enabled by surface oxidation layers of transition-metal phosphide and chalcogenide materials. We for the first time find that CoP nanoparticles strongly adsorb polysulfides because their natural oxidation (forming Co–O–P-like species) activates the surface Co sites for binding polysulfides via strong Co–S bonding. With a surface oxidation layer capable of confining polysulfides and an inner core suitable for conducting electrons, the CoP nanoparticles are thus a desirable candidate for stabilizing and improving the performance of sulfur cathodes in lithium–sulfur batteries. We demonstrate that sulfur electrodes that hold a high mass loading of 7 mg cm^–2 and a high areal capacity of 5.6 mAh cm^–2 can be stably cycled for 200 cycles. We further reveal that this new surface oxidation-induced polysulfide-binding scheme applies to a series of transition-metal phosphide and chalcogenide materials and can explain their stabilizing effects for lithium–sulfur batteries

Coupled Metal/Oxide Catalysts with Tunable Product Selectivity for Electrocatalytic CO₂ Reduction

One major challenge to the electrochemical conversion of CO₂ to useful fuels and chemical products is the lack of efficient catalysts that can selectively direct the reaction to one desirable product and avoid the other possible side products. Making use of strong metal/oxide interactions has recently been demonstrated to be effective in enhancing electrocatalysis in the liquid phase. Here, we report one of the first systematic studies on composition-dependent influences of metal/oxide interactions on electrocatalytic CO₂ reduction, utilizing Cu/SnO_<i>x</i> heterostructured nanoparticles supported on carbon nanotubes (CNTs) as a model catalyst system. By adjusting the Cu/Sn ratio in the catalyst material structure, we can tune the products of the CO₂ electrocatalytic reduction reaction from hydrocarbon-favorable to CO-selective to formic acid-dominant. In the Cu-rich regime, SnO_<i>x</i> dramatically alters the catalytic behavior of Cu. The Cu/SnO_<i>x</i>–CNT catalyst containing 6.2% of SnO_<i>x</i> converts CO₂ to CO with a high faradaic efficiency (FE) of 89% and a <i>j</i>_CO of 11.3 mA·cm^–2 at −0.99 V versus reversible hydrogen electrode, in stark contrast to the Cu–CNT catalyst on which ethylene and methane are the main products for CO₂ reduction. In the Sn-rich regime, Cu modifies the catalytic properties of SnO_<i>x</i>. The Cu/SnO_<i>x</i>–CNT catalyst containing 30.2% of SnO_<i>x</i> reduces CO₂ to formic acid with an FE of 77% and a <i>j</i>_HCOOH of 4.0 mA·cm^–2 at −0.99 V, outperforming the SnO_<i>x</i>–CNT catalyst which only converts CO₂ to formic acid in an FE of 48%

Electroreduction of CO₂ Catalyzed by a Heterogenized Zn–Porphyrin Complex with a Redox-Innocent Metal Center

Transition-metal-based molecular complexes are a class of catalyst materials for electrochemical CO₂ reduction to CO that can be rationally designed to deliver high catalytic performance. One common mechanistic feature of these electrocatalysts developed thus far is an electrogenerated reduced metal center associated with catalytic CO₂ reduction. Here we report a heterogenized zinc–porphyrin complex (zinc­(II) 5,10,15,20-tetramesitylporphyrin) as an electrocatalyst that delivers a turnover frequency as high as 14.4 site^–1 s^–1 and a Faradaic efficiency as high as 95% for CO₂ electroreduction to CO at −1.7 V vs the standard hydrogen electrode in an organic/water mixed electrolyte. While the Zn center is critical to the observed catalysis, in situ and operando X-ray absorption spectroscopic studies reveal that it is redox-innocent throughout the potential range. Cyclic voltammetry indicates that the porphyrin ligand may act as a redox mediator. Chemical reduction of the zinc–porphyrin complex further confirms that the reduction is ligand-based and the reduced species can react with CO₂. This represents the first example of a transition-metal complex for CO₂ electroreduction catalysis with its metal center being redox-innocent under working conditions

Strong Metal–Phosphide Interactions in Core–Shell Geometry for Enhanced Electrocatalysis

Rational design of multicomponent material structures with strong interfacial interactions enabling enhanced electrocatalysis represents an attractive but underdeveloped paradigm for creating better catalysts for important electrochemical energy conversion reactions. In this work, we report metal–phosphide core–shell nanostructures as a new model electrocatalyst material system where the surface electronic states of the shell phosphide and its interactions with reaction intermediates can be effectively influenced by the core metal to achieve higher catalytic activity. The strategy is demonstrated by the design and synthesis of iron–iron phosphide (Fe@FeP) core–shell nanoparticles on carbon nanotubes (CNTs) where we find that the electronic interactions between the metal and the phosphide components increase the binding strength of hydrogen adatoms toward the optimum. As a consequence, the Fe@FeP/CNT material exhibits exceptional catalytic activity for the hydrogen evolution reaction, only requiring overpotentials of 53–110 mV to reach catalytic current densities of 10–100 mA cm^–2