Extremely low-mass white dwarfs (ELM WDs) are helium WDs with a mass less than ∼0.3 M . Most ELM WDs are found in double degenerates (DDs) in the ELM Survey led by Brown and Kilic. These systems are ...supposed to be significant gravitational-wave sources in the mHz frequency. In this paper, we first analyzed the observational characteristics of ELM WDs and found that there are two distinct groups in the ELM WD mass and orbital period plane, indicating two different formation scenarios of such objects, i.e., a stable Roche lobe overflow channel (RL channel) and common envelope ejection channel (CE channel). We then systematically investigated the formation of ELM WDs in DDs by a combination of detailed binary evolution calculation and binary population synthesis. Our study shows that the majority of ELM WDs with mass less than 0.22 M are formed from the RL channel. The most common progenitor mass in this way is in the range of 1.15-1.45 M , and the resulting ELM WDs have a peak around 0.18 M when selection effects are taken into account, consistent with observations. The ELM WDs with a mass larger than 0.22 M are more likely to be from the CE channel and have a peak of ELM WD mass around 0.25 M , which needs to be confirmed by future observations. By assuming a constant star formation rate of 2 M yr−1 for a Milky Way-like galaxy, the birth rate and local density are 5 × 10−4 yr−1 and 1500 kpc−3, respectively, for DDs with an ELM WD mass less than 0.25 M .
The development of metal‐N‐C materials as efficient non‐precious metal (NPM) catalysts for catalysing the oxygen reduction reaction (ORR) as alternatives to platinum is important for the practical ...use of proton exchange membrane fuel cells (PEMFCs). However, metal‐N‐C materials have high structural heterogeneity. As a result of their high‐temperature synthesis they often consist of metal‐Nx sites and graphene‐encapsulated metal nanoparticles. Thus it is hard to identify the active structure of metal‐N‐C catalysts. Herein, we report a low‐temperature NH4Cl‐treatment to etch out graphene‐encapsulated nanoparticles from metal‐N‐C catalysts without destruction of co‐existing atomically dispersed metal‐Nx sites. Catalytic activity is much enhanced by this selective removal of metallic nanoparticles. Accordingly, we can confirm the spectator role of graphene‐encapsulated nanoparticles and the pivotal role of metal‐Nx sites in the metal‐N‐C materials for ORR in the acidic medium.
ORR inspiring: With a low‐temperature NH4Cl treatment graphene‐encapsulated nanoparticles (NPs) are etched out of metal‐N‐C catalysts. Removing these metallic NPs greatly enhances the catalytic oxygen reduction reaction (ORR) activity allowing the real catalytic centres to be identified.
Single‐ion conducting polymer electrolytes are considered particularly attractive for realizing high‐performance solid‐state lithium‐metal batteries. Herein, a polysiloxane‐based single‐ion conductor ...(PSiO) is investigated. The synthesis is performed via a simple thiol‐ene reaction, yielding flexible and self‐standing polymer electrolyte membranes (PSiOM) when blended with poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVdF‐HFP). When incorporating 57 wt% of organic carbonates, these polymer membranes provide a Li+ conductivity of >0.4 mS cm−1 at 20 °C and a wide electrochemical stability window of more than 4.8 V. This excellent electrochemical stability allows for the highly reversible cycling of symmetric Li||Li cells as well as high‐energy Li||LiNi0.6Mn0.2Co0.2O2 (NMC622) and Li||LiNi0.8Mn0.1Co0.1O2 (NMC811) cells for several hundred cycles at relatively high discharge and charge rates. Remarkably, Li||NMC811 cells with high mass loading cathodes provide more than 76% capacity retention at a high current density of 1.44 mA cm−2, thus rendering this polymer electrolyte suitable for high‐performance battery applications.
A polysiloxane‐based single‐ion conducting polymer electrolyte comprised of organic carbonates enables a high Li+ conductivity of >0.4 mS cm−1 (20 °C) and provides stable cycling in high‐energy Li||NMC622 and Li||NMC811 cells for several hundred cycles. Remarkably, the polymer‐based electrolyte can withstand current densities exceeding 5 mA cm–2 at reasonable specific capacities.
Metal-support interaction is of great significance for catalysis as it can induce charge transfer between metal and support, tame electronic structure of supported metals, impact adsorption energy of ...reaction intermediates, and eventually change the catalytic performance. Here, we report the metal size-dependent charge transfer reversal, that is, electrons transfer from platinum single atoms to sulfur-doped carbons and the carbon supports conversely donate electrons to Pt when their size is expanded to ~1.5 nm cluster. The electron-enriched Pt nanoclusters are far more active than electron-deficient Pt single atoms for catalyzing hydrogen evolution reaction, exhibiting only 11 mV overpotential at 10 mA cm
and a high mass activity of 26.1 A mg
at 20 mV, which is 38 times greater than that of commercial Pt/C. Our work manifests that the manipulation of metal size-dependent charge transfer between metal and support opens new avenues for developing high-active catalysts.
Recently, heteroatom‐doped three‐dimensional (3D) nanostructured carbon materials have attracted immense interest because of their great potential in various applications. Hence, it is highly ...desirable to exploit a simple, renewable, scalable, multifunctional, and general strategy to engineer 3D heteroatom‐doped carbon nanomaterials. Herein, a simple, eco‐friendly, general, and effective way to fabricate 3D heteroatom‐doped carbon nanofiber networks on a large scale is reported. Using this method, 3D P‐doped, N,P‐co‐doped, and B,P‐co‐doped carbon nanofiber networks are successfully fabricated by the pyrolysis of bacterial cellulose immersed in H3PO4, NH4H2PO4, and H3BO3/H3PO4 aqueous solution, respectively. Moreover, the as‐prepared N,P‐co‐doped carbon nanofibers exhibit good supercapacitive performance.
A simple, efficient, and general approach is developed for preparing cost‐effective, three‐dimensional, and large‐scale heteroatom‐doped carbon nanofibers, such as P‐doped, N,P‐co‐doped, and B,P‐co‐doped carbon nanofibers, by pyrolyzing bacterial cellulose (BC) previously immersed in H3PO4, NH4H2PO4, and H3BO3/H3PO4, respectively. Moreover, the as‐prepared N,P‐co‐doped carbon nanofibers exhibit good supercapacitive performance.
Abstract
Metal single-atom catalysts (M-SACs) have emerged as an attractive concept for promoting heterogeneous reactions, but the synthesis of high-loading M-SACs remains a challenge. Here, we ...report a multilayer stabilization strategy for constructing M-SACs in nitrogen-, sulfur- and fluorine-co-doped graphitized carbons (M = Fe, Co, Ru, Ir and Pt). Metal precursors are embedded into perfluorotetradecanoic acid multilayers and are further coated with polypyrrole prior to pyrolysis. Aggregation of the metals is thus efficiently inhibited to achieve M-SACs with a high metal loading (~16 wt%). Fe-SAC serves as an efficient oxygen reduction catalyst with half-wave potentials of 0.91 and 0.82 V (versus reversible hydrogen electrode) in alkaline and acid solutions, respectively. Moreover, as an air electrode in zinc–air batteries, Fe-SAC demonstrates a large peak power density of 247.7 mW cm
−2
and superior long-term stability
.
Our versatile method paves an effective way to develop high-loading M-SACs for various applications.
Heterostructures exhibit considerable potential in the field of energy conversion due to their excellent interfacial charge states in tuning the electronic properties of different components to ...promote catalytic activity. However, the rational preparation of heterostructures with highly active heterosurfaces remains a challenge because of the difficulty in component tuning, morphology control, and active site determination. Herein, a novel heterostructure based on a combination of RuMo nanoalloys and hexagonal N‐doped carbon nanosheets is designed and synthesized. In this protocol, metal‐containing anions and layered double hydroxides are employed to control the components and morphology of heterostructures, respectively. Accordingly, the as‐made RuMo‐nanoalloys‐embedded hexagonal porous carbon nanosheets are promising for the hydrogen evolution reaction (HER), resulting in an extremely small overpotential (18 mV), an ultralow Tafel slope (25 mV dec−1), and a high turnover frequency (3.57 H2 s−1) in alkaline media, outperforming current Ru‐based electrocatalysts. First‐principle calculations based on typical 2D N‐doped carbon/RuMo nanoalloys heterostructures demonstrate that introducing N and Mo atoms into C and Ru lattices, respectively, triggers electron accumulation/depletion regions at the heterosurface and consequently reduces the energy barrier for the HER. This work presents a convenient method for rational fabrication of carbon–metal heterostructures for highly efficient electrocatalysis.
A novel heterostructure based on uniform RuMo nanoalloys and hexagonal N‐doped carbon nanosheets is prepared through a combination of hard template and anion‐exchange methods. The obtained material exhibits excellent electrocatalytic activity for the hydrogen evolution reaction. Theoretical calculation confirms that the heterosurfaces play a crucial role in accelerating the hydrogen evolution activity.
Exploring low‐cost and high‐performance nonprecious metal catalysts (NPMCs) for oxygen reduction reaction (ORR) in fuel cells and metal–air batteries is crucial for the commercialization of these ...energy conversion and storage devices. Here we report a novel NPMC consisting of Fe3C nanoparticles encapsulated in mesoporous Fe‐N‐doped carbon nanofibers, which is synthesized by a cost‐effective method using carbonaceous nanofibers, pyrrole, and FeCl3 as precursors. The electrocatalyst exhibits outstanding ORR activity (onset potential of −0.02 V and half‐wave potential of −0.140 V) closely comparable to the state‐of‐the‐art Pt/C catalyst in alkaline media, and good ORR activity in acidic media, which is among the highest reported activities of NPMCs.
Nanocomposite electrocatalyst: A high‐performance electrocatalyst for the oxygen reduction reaction (ORR) is based on Fe3C nanoparticles encapsulated in mesoporous Fe‐N‐doped carbon nanofibers. It can be synthesized from low‐cost and abundant precursors and exhibits excellent electrocatalytic performance for the ORR in both alkaline and acidic media.
Thirsty fibers: The aerogels described in the title can be fabricated in large scale by using a low‐cost biomass, bacterial cellulose, as a precursor, which can be produced at industrial level in a ...microbial fermentation process. The carbon nanofiber aerogels (black pieces in picture) exhibit superior absorption capacity for organic solvents (red solution) and high potential for pressure sensing.
Abstract
Supported metal nanoclusters consisting of several dozen atoms are highly attractive for heterogeneous catalysis with unique catalytic properties. However, the metal nanocluster catalysts ...face the challenges of thermal sintering and consequent deactivation owing to the loss of metal surface areas particularly in the applications of high-temperature reactions. Here, we report that sulfur—a documented poison reagent for metal catalysts—when doped in a carbon matrix can stabilize ~1 nanometer metal nanoclusters (Pt, Ru, Rh, Os, and Ir) at high temperatures up to 700 °C. We find that the enhanced adhesion strength between metal nanoclusters and the sulfur-doped carbon support, which arises from the interfacial metal-sulfur bonding, greatly retards both metal atom diffusion and nanocluster migration. In catalyzing propane dehydrogenation at 550 °C, the sulfur-doped carbon supported Pt nanocluster catalyst with interfacial electronic effects exhibits higher selectivity to propene as well as more stable durability than sulfur-free carbon supported catalysts.
PDF
