Hexagonal birnessite is the most ubiquitous manganese oxide in geological environments. It is often highly enriched in trace metal ions such as Ni and plays an important role in metal(loids) geochemistry . Nanostructured birnessites containing different amounts of Ni were synthesized by addition of Ni 2+ to initial reactants. Powder X-ray diffraction (XRD), element analysis, field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), X-ray absorption spectroscopy (XAS) and isothermal adsorption and oxidation of metal(loids) were carried out to investigate the effects of Ni doping on the substructure and physicochemical properties of birnessite, and Ni crystal chemistry in birnessite. These Ni-rich birnessites have Ni contents as high as 2.99% (Ni5) and 6.08% (Ni10) in weight. EXAFS results show that Ni5 has 23.7% of the total Ni (0.71 wt.%) and Ni10 has 34.5% of the total Ni (2.10 wt.%) in Mn octahedral layer with the remaining Ni located at vacancies and edge sites. The Ni-rich birnessites have weaker crystallinity and thermal stability, fewer layers stacked along the c axis, ∼1.5–2.7 times larger surfaces areas, and a higher Mn average oxidation numbers (AONs) compared to the birnessite without Ni. Additionally, the doping of Ni during birnessite crystallization enhances the formation of vacancies in the layer; however, adsorption capacities for Pb 2+ and Zn 2+ by these Ni-rich birnessites are reduced, mainly because of vacancies and edge sites occupation by a large amount of Ni. The Ni-rich birnessites exhibit much higher oxidation capability and can completely oxidize As(III) in solution at rapid initial reaction rates under the experimental condition. The results indicate that incorporation of Ni into the natural birnessite in ferromanganese nodules may be achieved both by direct coprecipitation with Mn to build the layers and migration over time from adsorbed Ni on the surface into the layer structure. It is also implied that Ni doping in birnessite has great impact on the geochemical behaviors of heavy metals, either in adsorption or oxidation reactions.
Xinyue Luo,Hongqin Ma,Junyu Gao,Luqi Yu,Xingpeng Gu,Jiahao Liu
The development of an effective and inexpensive electrocatalyst for the oxygen evolution reaction (OER) is the key to achieve high hydrogen production since water splitting has sluggish OER kinetics. Herein, a composite of nickel-rich nickel nitride (NR-Ni3N) with graphitic carbon nitrides (GCNs) was prepared for the electrocatalytic OER, showing a low overpotential of ∼290 mV, a small Tafel slope of ∼70 mV dec–1 at 10 mA cm–2 current density, and low loss of activity after 36 h of test in an alkaline electrolyte, outperforming the pure Ni3N and even the noble catalyst RuO2. The characterization results revealed that the defective GCNs can promote the generation of a Ni-rich region in Ni3N in the nitridation process, and the high conductivity of the formed robust Ni–Ni3N heterojunction and its strong synergistic effect with GCNs explain the excellent OER electrocatalytic performances of NR-Ni3N/GCNs. This work provides a novel feasible strategy for the development of highly active OER electrocatalysts.
The layered lithium nickel-rich cathodes have become one of the most promising candidates for next-generation lithium-ion batteries with high energy density , However, the nickel-rich cathode suffers from a rapid decay of capacity due to their intrinsic instability and undesirably residual lithium compounds originated from LiNiO 2 structural changes on the surface, hindering large-scale commercial application. Herein, a continuous and uniform oxygen vacancy enriched-MoO 3-x coating was successfully synthesized and reacted residual lithium compounds as a functional layer on the cathode. Such chemically reconstructed coating (Li 2 MoO 4 ) strategy endows the 1 wt% MoO 3-x coated NCA cathode with a fast Li + ions transport and enhanced structural stability , leading to an outstanding discharge capacity of 214.0 mAh g −1 at 0.1C (1C = 270 mA g −1 ), a better capacity retention of 80.3 % after 120 cycles at 0.5C, and an excellent high rate capability of 120 mAh g −1 at 5C cycled in voltage range of 2.75 V–4.3 V. This work provides an effective strategy to modulate surface structure towards advanced Ni-rich cathodes for high-performance lithium-ion batteries.
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