The development of precious‐metal alternative electrocatalysts for oxygen reduction reaction (ORR) is highly desired for a variety of fuel cells, and single atom catalysts (SACs) have been envisaged ...to be the promising choice. However, there remains challenges in the synthesis of high metal loading SACs (>5 wt.%), thus limiting their electrocatalytic performance. Herein, a facile self‐sacrificing template strategy is developed for fabricating Co single atoms along with Co atomic clusters co‐anchored on porous‐rich nitrogen‐doped graphene (Co SAs/AC@NG), which is implemented by the pyrolysis of dicyandiamide with the formation of layered g‐C3N4 as sacrificed templates, providing rich anchoring sites to achieve high Co loading up to 14.0 wt.% in Co SAs/AC@NG. Experiments combined with density functional theory calculations reveal that the co‐existence of Co single atoms and clusters with underlying nitrogen doped carbon in the optimized Co40SAs/AC@NG synergistically contributes to the enhanced electrocatalysis for ORR, which outperforms the state‐of‐the‐art Pt/C catalysts with presenting a high half‐wave potential (E1/2 = 0.890 V) and robust long‐term stability. Moreover, the Co40SAs/AC@NG presents excellent performance in Zn–air battery with a high‐peak power density (221 mW cm−2) and strong cycling stability, demonstrating great potential for energy storage applications.
High‐loading Co single atoms and Co atomic clusters co‐anchored on porous‐rich nitrogen‐doped graphene (Co SAs/AC@NG) is constructed via a facile self‐sacrificing template strategy. The Co40SAs/AC@NG catalyst demonstrates remarkable performance with a half‐wave potential of 0.890 V for oxygen reduction reaction and a large power density of 221 mW cm−2 toward Zn–air battery.
Production of hydrogen by electrochemical water splitting has been hindered by the high cost of precious metal catalysts, such as Pt, for the hydrogen evolution reaction (HER). In this work, novel ...hierarchical β‐Mo2C nanotubes constructed from porous nanosheets have been fabricated and investigated as a high‐performance and low‐cost electrocatalyst for HER. An unusual template‐engaged strategy has been utilized to controllably synthesize Mo‐polydopamine nanotubes, which are further converted into hierarchical β‐Mo2C nanotubes by direct carburization at high temperature. Benefitting from several structural advantages including ultrafine primary nanocrystallites, large exposed surface, fast charge transfer, and unique tubular structure, the as‐prepared hierarchical β‐Mo2C nanotubes exhibit excellent electrocatalytic performance for HER with small overpotential in both acidic and basic conditions, as well as remarkable stability.
From the same sheet: Hierarchical β‐Mo2C nanotubes constructed of ultrathin nanosheets are designed and synthesized. Benefitting from ultra‐small primary nanocrystallites, a large exposed surface, fast charge transfer, and unique tubular structure, the as‐prepared hierarchical β‐Mo2C nanotubes exhibit excellent electrocatalytic performance for the hydrogen evolution reaction.
Exploring highly active and cost‐efficient single‐atom catalysts (SACs) for oxygen reduction reaction (ORR) is critical for the large‐scale application of Zn–air battery. Herein, density functional ...theory (DFT) calculations predict that the intrinsic ORR activity of the active metal of SACs follows the trend of Co > Fe > Ni ≈ Cu, in which Co SACs possess the best ORR activity due to its optimized spin density. Guided by DFT calculations, four kinds of transition metal single atoms embedded in 3D porous nitrogen‐doped carbon nanosheets (MSAs@PNCN, M = Co, Ni, Fe, Cu) are synthesized via a facile NaCl‐template assisted strategy. The resulting MSAs@PNCN displays ORR activity trend in lines with the theoretical predictions, and the Co SAs@PNCN exhibits the best ORR activity (E1/2 = 0.851 V), being comparable to that of Pt/C under alkaline conditions. X‐ray absorption fine structure (XAFS) spectra verify the atomically dispersed Co‐N4 sites are the catalytically active sites. The highly active CoN4 sites and the unique 3D porous structure contribute to the outstanding ORR performance of Co SAs@PNCN. Furthermore, the Co SAs@PNCN catalyst is employed as cathode in Zn–air battery, which can deliver a large power density of 220 mW cm–2 and maintain robust cycling stability over 530 cycles.
Four kinds of transition metal single atoms embedded in 3D porous nitrogen‐doped carbon nanosheets (MSAs@PNCN, M = Co, Ni, Fe, Cu) are constructed via a facile NaCl‐template assisted strategy. The Co SAs@PNCN catalysts demonstrate remarkable performance with a half‐wave potential of 0.851 V for oxygen reduction reaction and a large power density of 220 mW cm–2 toward Zn–air battery.
Hierarchical Fe3O4 hollow spheres constructed by nanosheets are obtained from solvothermally synthesized Fe–glycerate hollow spheres. With the unique structural features, these hierarchical Fe3O4 ...hollow spheres exhibit excellent electrochemical lithium‐storage performance.
Transition metal single atoms anchored on nitrogen‐doped carbon (M‐N‐C) matrix with M‐N‐C active sites have shown to be promising catalysts for both hydrogen evolution reaction (HER) and oxygen ...reduction reaction (ORR). Herein, a hybrid catalyst with low‐level loading of atomic Pt and Co species encapsulated in nitrogen‐doped graphene (Pt@CoN4‐G) is developed. The Pt@CoN4‐G shows low overpotential for HER in wide‐pH electrolyte and manifests improved mass activity with almost eight times greater than that of Pt/C at an overpotential of 50 mV. The Pt@CoN4‐G also exhibits a top‐level ORR activity (half‐wave potential, E1/2 = 0.893 V) and robust stability (>200 h) in alkaline medium. Using theoretical calculations and comprehensive characterizations , the strong metal–support interactions between Pt species and CoN4‐G support and synergistical cooperation of multiple active sites are clarified. A flow alkali‐Al/acid hybrid fuel cell using Pt@CoN4‐G as cathode catalyst delivers a large power density of 222 mW cm−2 with excellent stability to achieve simultaneously hydrogen evolution and electricity generation. In addition, Pt@CoN4‐G endows a flow Zn‐air battery with high power density (316 mW cm−2), good stability under large current density (>100 h at 100 mA cm−2), and long cycle life (over 600 h at 5 mA cm−2).
A hybrid catalyst with low‐level loading of atomic Pt and Co species encapsulated in nitrogen‐doped graphene (Pt@CoN4‐G) is developed. The Pt@CoN4‐G catalyst shows the enhanced hydrogen evolution reaction and oxygen reduction reaction activities due to the strong metal–support interactions between Pt species and CoN4‐G support, and synergistical cooperation of multiple active sites, including CoN4, Pt clusters, and PtN2.
Transition metal–nitrogen–carbon (TM–N–C) catalysts have been intensely investigated to tackle the sluggish oxygen reduction reactions (ORRs), but insufficient accessibility of the active sites ...limits their performance. Here, by using solid ZIF‐L nanorods as self‐sacrifice templates, a ZIF‐phase‐transition strategy is developed to fabricate ZIF‐8 hollow nanorods with open cavities, which can be subsequently converted to atomically dispersed Fe‐N‐C hollow nanorods (denoted as Fe1–N–C HNRs) through rational carbonization and following fixation of iron atoms. The microstructure observation and X‐ray absorption fine structure analysis confirm abundant Fe–N4 active sites are evenly distributed in the carbon skeleton. Thanks to the highly accessible Fe‐N4 active sites provided by the highly porous and open carbon hollow architecture, the Fe1‐N‐C HNRs exhibit superior ORR activity and stability in alkaline and acidic electrolytes with very positive half‐wave potentials of 0.91 and 0.8 V versus RHE, respectively, both of which surpass those of commercial Pt/C. Remarkably, the dynamic current density (JK) of Fe1‐N‐C HNRs at 0.85 V versus RHE in alkaline media delivers a record value of 148 mA cm−2, 21 times higher than that of Pt/C. The assembled Zn‐air battery using Fe1–N–C HNRs as cathode catalyst exhibits a high peak power density of 208 mW cm−2.
By using solid ZIF‐L nanorods as self‐sacrifice templates, a unique ZIF‐phase‐transition strategy is developed to fabricate atomically dispersed Fe–N–C hollow nanorods (Fe1–N–C HNRs) with highly open architecture and abundant exposed Fe–N4 active sites, which can be utilized as efficient oxygen reduction reaction electrocatalysts in both alkaline and acid conditions.
It is challenging yet promising to design highly accessible N‐doped carbon skeletons to fully expose the active sites inside single‐atom catalysts. Herein, mesoporous N‐doped carbon hollow spheres ...with regulatable through‐pore size can be formulated by a simple sequential synthesis procedure, in which the condensed SiO2 is acted as removable dual‐templates to produce both hollow interiors and through‐pores, meanwhile, the co‐condensed polydopamine shell is served as N‐doped carbon precursor. After that, Fe─N─C hollow spheres (HSs) with highly accessible active sites can be obtained after rationally implanting Fe single‐atoms. Microstructural analysis and X‐ray absorption fine structure analysis reveal that high‐density Fe─N4 active sites together with tiny Fe clusters are uniformly distributed on the mesoporous carbon skeleton with abundant through‐pores. Benefitted from the highly accessible Fe─N4 active sites arising from the unique through‐pore architecture, the Fe─N─C HSs demonstrate excellent oxygen reduction reaction (ORR) performance in alkaline media with a half‐wave potential up to 0.90 V versus RHE and remarkable stability, both exceeding the commercial Pt/C. When employing Fe─N─C HSs as the air‐cathode catalysts, the assembled Zn–air batteries deliver a high peak power density of 204 mW cm−2 and stable discharging voltage plateau over 140 h.
Using two types of orthosilicates with different hydrolysis rates as dual‐silica sources and dopamine as the N‐doped carbon sources, a sequential synthesis procedure referred to the classic Stöber method is developed to fabricate N‐doped carbon hollow spheres with abundant unusual through‐pores. The formed N‐doped carbon hollow spheres can act as a favorable host to isolate Fe single‐atoms.
The low energy efficiency and limited cycling life of rechargeable Zn–air batteries (ZABs) arising from the sluggish oxygen reduction/evolution reactions (ORR/OERs) severely hinder their commercial ...deployment. Herein, a zeolitic imidazolate framework (ZIF)‐derived strategy associated with subsequent thermal fixing treatment is proposed to fabricate dual‐atom CoFe─N─C nanorods (Co1Fe1─N─C NRs) containing atomically dispersed bimetallic Co/Fe sites, which can promote the energy efficiency and cyclability of ZABs simultaneously by introducing the low‐potential oxidation redox reactions. Compared to the mono‐metallic nanorods, Co1Fe1─N─C NRs exhibit remarkable ORR performance including a positive half‐wave potential of 0.933 V versus reversible hydrogen electrode (RHE) in alkaline electrolyte. Surprisingly, after introducing the potassium iodide (KI) additive, the oxidation overpotential of Co1Fe1─N─C NRs to reach 10 mA cm−2 can be significantly reduced by 395 mV compared to the conventional destructive OER. Theoretical calculations show that the markedly decreased overpotential of iodide oxidation can be ascribed to the synergistic effects of neighboring Co─Fe diatomic sites as the unique adsorption sites. Overall, aqueous ZABs assembled with Co1Fe1─N─C NRs and KI as the air–cathode catalyst and electrolyte additive, respectively, can deliver a low charging voltage of 1.76 V and ultralong cycling stability of over 230 h with a high energy efficiency of ≈68%.
A zeolitic imidazolate framework (ZIF)derived strategy associated with subsequent thermal fixing treatment is proposed to fabricate dual‐atom CoFe─N─C nanorods containing atomically dispersed bimetallic Co/Fe sites, which can be used as bifunctional air–cathode catalysts to boost the energy efficiency and cyclability of Zn–air batteries simultaneously by introducing low‐potential oxidation redox reactions.
Graphene-like two-dimensional layered materials have attracted quite a lot of interest because of their sizable band gaps and potential applications. In this work, monodisperse tin disulfide (SnS2) ...nanosheets were successfully prepared by a simple solvothermal procedure in the presence of polyvinylpyrrolidone (PVP). Large PVP molecules absorbing on (001) facets of SnS2 would inhibit crystal growth along 001 orientation and protect the product from agglomeration. The obtained SnS2 nanosheets have diameters of ca. 0.8–1 μm and thicknesses of ca. 22 nm. Different experiment parameters were carried out to investigate the transformation of phase and morphology. The formation mechanism was proposed according to the time-dependent experiments. SnS2 nanosheets exhibit high photocatalytic H2 evolution activity of 1.06 mmol h–1 g–1 under simulated sunlight irradiation, much higher than that of SnS2 with different morphologies and P25-TiO2. Moreover, the as-obtained SnS2 nanosheets show excellent photoelectrochemical response performance in visible-light region.
Tibet’s ancient topography and its role in climatic and biotic evolution remain speculative due to a paucity of quantitative surface-height measurements through time and space, and sparse fossil ...records. However, newly discovered fossils from a present elevation of ∼4,850 m in central Tibet improve substantially our knowledge of the ancient Tibetan environment. The 70 plant fossil taxa so far recovered include the first occurrences of several modern Asian lineages and represent a Middle Eocene (∼47 Mya) humid subtropical ecosystem. The fossils not only record the diverse composition of the ancient Tibetan biota, but also allow us to constrain the Middle Eocene land surface height in central Tibet to ∼1,500 ± 900 m, and quantify the prevailing thermal and hydrological regime. This “Shangri-La”–like ecosystem experienced monsoon seasonality with a mean annual temperature of ∼19 °C, and frosts were rare. It contained few Gondwanan taxa, yet was compositionally similar to contemporaneous floras in both North America and Europe. Our discovery quantifies a key part of Tibetan Paleogene topography and climate, and highlights the importance of Tibet in regard to the origin of modern Asian plant species and the evolution of global biodiversity.
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