Cx Shang

Mechanical alloying and electronic simulations of (MgH2+M) systems (M=Al, Ti, Fe, Ni, Cu and Nb) for hydrogen storage

Abstract Mg-based alloys are promising candidates for hydrogen storage applications. Here, mechanical alloying (MA) was used to process powder mixtures of MgH2 with 8 mol % M (M=Al, Ti, Fe, Ni, Cu and Nb) in order to modify hydrogen storage properties of the Mg hydride. Electronic simulations of the systems were carried out to clarify the mechanisms of the alloy effects. X-ray diffraction (XRD) of the milled samples revealed the formation of new phases: a bcc solid solution phase for the (MgH2+Nb) mixture; TiH2 phase for the (MgH2+Ti); and MgCu2 phase for the (MgH2+Cu). For all the mixtures, a high-pressure phase, γ-MgH2, was also identified after mechanical alloying. Further qualitative and quantitative phase analyses were carried out using the Rietveld method. Scanning electron microscopy (SEM) of the milled powder clearly showed substantial particle size reduction after milling. Dehydrogenation at 300°C under vacuum shows that the (MgH2+Ni) mixture gives the highest level of hydrogen desorption and the most rapid kinetics, followed by MgH2 with Al, Fe, Nb, Ti and Cu. Theoretical predictions show that the (MgH2+Cu) system is the most unstable, followed by (MgH2+Ni), (MgH2+Fe), (MgH2+Al), (MgH2+Nb), (MgH2+Ti). The predicted alloying effects on the stability of MgH2 generally agree with the experimentally observed change in the hydrogen desorption capacity. The differences were discussed in the text.

Active sites engineering leads to exceptional ORR and OER bifunctionality in P,N Co-doped graphene frameworks

Bifunctional catalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are highly desirable for rechargeable metal–air batteries and regenerative fuel cells. However, the commercial oxygen electrocatalysts (mainly noble metal based) can only exhibit either ORR or OER activity and also suffer from inherent cost and stability issues. It remains challenging to achieve efficient ORR and OER bifunctionality on a single catalyst. Metal-free structures offer relatively large scope for this bifunctionality to be engineered within one catalyst, together with improved cost-effectiveness and durability. Herein, by closely coupled computational design and experimental development, highly effective bifunctionality was achieved in a phosphorus and nitrogen co-doped graphene framework (PNGF) – with both ORR and OER activities reaching the theoretical limits of metal-free catalysts, superior to their noble metal counterparts in both (bi)functionality and durability. In particular, with the identification of active P–N sites for OER and N-doped sites for ORR, we successfully intensified these sites by one-pot synthesis to tailor the PNGF. The resulting catalyst achieved an ORR potential of 0.845 V vs. RHE at 3 mA cm−2 and an OER potential of 1.55 V vs. RHE at 10 mA cm−2. Its combined ORR and OER overpotential of 705 mV is much lower than those previously reported for metal-free bifunctional catalysts.

Structural stability of mechanically alloyed (Mg+10Nb) and (MgH2+10Nb) powder mixtures

In order to improve the hydrogen storage characteristics of magnesium, both chemical alloying by Nb and mechanical alloying (MA) of (Mg+10 wt.%Nb) and (MgH2+10 wt.%Nb) powder mixtures were investigated, with particular attention paid to their structural stability. Extensive powder refinement was noted for both compositions within 20 h of milling at 250 rpm. Even nano-sized particles were generated in the hydride mixture. XRD and Rietveld analyses show the formation of a bcc phase in each case. The amount of the bcc phase increases with increasing milling time to the detriment of Nb. For the (Mg+10 wt.%Nb) mixture, it is confirmed that the newly formed phase is a bcc-(Nb,Mg) solid solution, with an extended solubility of Nb in Mg. However, for the (MgH2+10 wt.%Nb) powder mixture, the new bcc phase can be a Nb hydride (NbHx, x<1.0), or a bcc-(Nb,Mg) solid solution, or a (Nb,Mg)Hx solid solution, or even a mixture of the three.

Mesoporous Fe2O3 flakes of high aspect ratio encased within thin carbon skeleton for superior lithium-ion battery anodes

Reticulated mesoporous Fe2O3@C flakes, consisting of nanocrystalline α-Fe2O3 encased within a thin carbon skeleton, were synthesized using ferrocene as iron and carbon sources and a novel reaction agent of ammonium sulphate via a facile two-step heating route. Those flakes, which were several nanometers thick and 1–2 μm in diameter, showed high capacity, excellent cyclic stability and rate capability as an anode material for lithium-ion batteries (LIBs). An initial reversible capacity of 910 mA h g−1 at a discharge/charge current of 0.1 A g−1 was obtained, and the capacity showed a gradual increase during cycling, reaching a high capacity of 1080 mA h g−1 after 120 cycles. An in situ lithiation study by transmission electron microscopy showed that the reticulated mesopores and the ultrathin feature of the Fe2O3@C flakes can largely accommodate the mechanical stresses and volume expansion of Fe2O3 during lithiation and hence maintain their integrity and provide excellent properties. The thin carbon skeleton not only facilitates the electronic conduction, but also inhibits the aggregation of nanocrystalline Fe2O3 flakes. The facile fabrication method and the unique structure of the mesoporous Fe2O3@C flakes offer high performance for LIB anodes and great potential for other applications of Fe2O3.

Naturally derived porous carbon with selective metal- and/or nitrogen-doping for efficient CO2 capture and oxygen reduction

A heterogeneously porous “green carbon” structure was derived from abundant London plane leaves and shows excellent performance for both CO2 capture and Oxygen Reduction Reaction (ORR). The carbonised and KOH-activated carbon possesses a high level of micropores, a specific surface area exceeding 2000 m2 g−1 and a large pore volume of over 1 cm3 g−1, leading to an excellent CO2 uptake of 19.4 wt% under ambient conditions and fast four-electron transfer in an alkaline medium for ORR. Furthermore, XPS and X-ray analyses reveal well-dispersed metal elements (such as Mg and Ca) in the porous carbon, which are naturally doped and inherited from the leaf structure, and can help to enhance CO2 adsorption. On the other hand, these metal elements do not positively affect catalytic ORR performance. Hence, a purpose-specific cleaning approach after KOH activation, i.e. by water or acid, has been devised to obtain optimal functionalities for CO2 capture or ORR.

A hybrid Si@FeSiy/SiOx anode structure for high performance lithium-ion batteries via ammonia-assisted one-pot synthesis

Synthesised via planetary ball-milling of Si and Fe powders in an ammonia (NH3) environment, a hybrid Si@FeSiy/SiOx structure shows exceptional electrochemical properties for lithium-ion battery anodes, exhibiting a high initial capacity of 1150 mA h g−1 and a retention capacity of 880 mA h g−1 after 150 cycles at 100 mA g−1; and a capacity of 560 mA h g−1 at 4000 mA g−1. These are considerably high for carbon-free micro-/submicro-Si-based anodes. NH3 gradually turns into N2 and H2 during the synthesis, which facilitates the formation of highly conductive FeSiy (y = 1, 2) phases, whereas such phases were not formed in an Ar atmosphere. Milling for 20–40 h leads to partial decomposition of NH3 in the atmosphere, and a hybrid structure of a Si core of mixed nanocrystalline and amorphous Si domains, shelled by a relatively thick SiOx layer with embedded FeSi nanocrystallites. Milling for 60–100 h results in full decomposition of NH3 and a hybrid structure of a much-refined Si-rich core surrounded by a mantle of a relatively low level of SiOx and a higher level of FeSi2. The formation mechanisms of the SiOx and FeSiy phases are explored. The latter structure offers an optimum combination of the high capacity of a nanostructural Si core, relatively high electric conductivity of the FeSiy phase and high structural stability of a SiOx shell accommodating the volume change for high performance electrodes. The synthesis method is new and indispensable for the large-scale production of high-performance Si-based anode materials.

Direct mechanical synthesis and characterisation of Mg2Fe(Cu)H6

Abstract Direct synthesis of Mg 2 FeH 6 was carried out by mechanically alloying MgH 2 with Fe under both Ar and H 2 atmospheres . Both (3MgH 2 +Fe) and (4MgH 2 +Fe) mixtures were processed to improve the yield of Mg 2 FeH 6 . The (MgH 2 –Fe–Cu) system was also investigated to modify the properties of the synthesised compound. X-ray diffraction and Rietveld analysis were carried out to determine the phase evolution of the powder mixtures. Field emission SEM clearly showed substantial particle size reduction for the mixture milled under hydrogen. Thermogravimetry (TG) was employed to determine the dehydrogenation kinetics of the milled mixtures.

Amylose-Derived Macrohollow Core and Microporous Shell Carbon Spheres as Sulfur Host for Superior Lithium–Sulfur Battery Cathodes

Porous carbon can be tailored to great effect for electrochemical energy storage. In this study, we propose a novel structured spherical carbon with a macrohollow core and a microporous shell derived from a sustainable biomass, amylose, by a multistep pyrolysis route without chemical etching. This hierarchically porous carbon shows a particle distribution of 2-10 μm and a surface area of 672 m2 g-1. The structure is an effective host of sulfur for lithium-sulfur battery cathodes, which reduces the dissolution of polysulfides in the electrolyte and offers high electrical conductivity during discharge/charge cycling. The hierarchically porous carbon can hold 48 wt % sulfur in its porous structure. The S@C hybrid shows an initial capacity of 1490 mAh g-1 and retains a capacity of 798 mAh g-1 after 200 cycles at a discharge/charge rate of 0.1 C. A capacity of 487 mAh g-1 is obtained at a rate of 3 C. Both a one-step pyrolysis and a chemical-reagent-assisted pyrolysis are also assessed to obtain porous carbon from amylose, but the obtained carbon shows structures inferior for sulfur cathodes. The multistep pyrolysis and the resulting hierarchically porous carbon offer an effective approach to the engineering of biomass for energy storage. The micrometer-sized spherical S@C hybrid with different sizes is also favorable for high-tap density and hence the volumetric density of the batteries, opening up a wide scope for practical applications.

Hydrogen-terminated mesoporous silicon monoliths with huge surface area as alternative Si-based visible light-active photocatalysts

Silicon-based nanostructures and their related composites have drawn tremendous research interest in solar energy storage and conversion. Mesoporous silicon with a huge surface area of 400–900 m2 g−1 developed by electrochemical etching exhibits excellent photocatalytic ability and stability after 10 cycles in degrading methyl orange under visible light irradiation, owing to its unique mesoporous network, abundant surface hydrides and efficient light harvesting. This work showcases the profound effects of surface area, crystallinity, pore topology on charge migration/recombination and mass transportation. Therein the ordered 1D channel array has outperformed the interconnected 3D porous network by greatly accelerating the mass diffusion and enhancing the accessibility of the active sites on the extensive surfaces.

A mechanochemical synthesis of submicron-sized Li2S and a mesoporous Li2S/C hybrid for high performance lithium/sulfur battery cathodes

Lithium sulfide, Li2S, is a promising cathode material for lithium–sulfur batteries (LSBs), with a high theoretical capacity of 1166 mA h g−1. However, it suffers from low cycling stability, low-rate capability and high initial activation potential. In addition, commercially available Li2S is of high cost and of large size, over ten microns, which further exacerbate its shortcomings as a sulfur cathode. Exploring new approaches to fabricate small-sized Li2S of low cost and to achieve Li2S cathodes of high electrochemical performance is highly desired. This work reports a novel mechanochemical method for synthesizing Li2S of high purity and submicron size by ball-milling LiH with sulfur in an Ar atmosphere at room temperature. By further milling the as-synthesized Li2S with polyacrylonitrile (PAN) followed by carbonization of PAN at 1000 °C, a Li2S/C hybrid with nano-sized Li2S embedded in a mesoporous carbon matrix is achieved. The hybrid with Li2S as high as 74 wt% shows a high initial capacity of 971 mA h g−1 at 0.1C and retains a capacity of 570 mA h g−1 after 200 cycles as a cathode material for LSBs. A capacity of 610 mA h g−1 is obtained at 1C. The synthesis method of Li2S is facile, environmentally benign, and of high output and low cost. The present work opens a new route for the scalable fabrication of submicron-sized Li2S and for the development of high performance Li2S-based cathodes.