Absorbents with “tree-like” structures, which were composed of hollow porous carbon fibers (HPCFs) acting as “trunk” structures, carbon nanotubes (CNTs) as “branch” structures and magnetite (Fe3O4) ...nanoparticles playing the role of “fruit” structures were prepared by chemical vapor deposition technique and chemical reaction. Microwave reflection loss, permittivity and permeability of Fe3O4–CNTs–HPCFs composites were investigated in the frequency range of 2–18GHz. It was proven that prepared absorbents possessed the excellent electromagnetic wave absorbing performances. The bandwidth with a reflection loss less than −15dB covers a wide frequency range from 10.2 to 18GHz with the thickness of 1.5–3.0mm, and the minimum reflection loss is −50.9dB at 14.03GHz with a 2.5mm thick sample layer. Microwave absorbing mechanism of the Fe3O4–CNTs–HPCFs composites is concluded as dielectric polarization and the synergetic interactions exist between Fe3O4 and CNTs–HPCFs.
Single‐atom cobalt‐based CoNC are promising low‐cost electrocatalysts for oxygen reduction reaction (ORR). However, further increasing the single cobalt‐based active sites and the ORR activity ...remain a major challenge. Herein, an acetate (OAc) assisted metal–organic framework (MOF) structure‐engineering strategy is developed to synthesize hierarchical accordion‐like MOF with higher loading amount and better spatial isolation of Co and much higher yield when compared with widely reported polyhedron MOF. After pyrolysis, the accordion‐structured CoNC (CoNC (A)) is loaded with denser CoN4 active sites (Co: 2.88 wt%), approximately twice that of Co in the CoNC reported. The presence of OAc in MOF also induces the generation of big pores (5–50 nm) for improving the accessibility of active sites and mass transfer during catalytic reactions. Consequently, the CoNC (A) catalyst shows an admirable ORR activity with a E1/2 of 0.89 V (40 mV better than Pt/C) in alkaline electrolytes, outstanding durability, and absolute tolerance to methanol in both alkaline and acidic media. The CoNC‐based Zn‐air battery exhibits a high specific capacity (976 mAh g−1Zn), power density (158 mW cm−2), rate capability, and long‐term stability. This work demonstrates a reliable approach to construct single atom doped carbon catalysts with denser accessible active sites through MOF structure engineering.
A new accordion‐like zeolitic‐imidazole framework (ZIF) with high Co ion loading amount and dispersity is prepared by metal–organic framework (MOF) structure engineering. The acetate (OAc) plays an important role in stabilization of more Co ions and formation of large pores during carbonization. The obtained CoNC (A) contains high density of accessible single atom Co‐based active sites and exhibits greatly enhanced oxygen reduction reaction (ORR) activity.
Ir‐based binary and ternary alloys are effective catalysts for the electrochemical oxygen evolution reaction (OER) in acidic solutions. Nevertheless, decreasing the Ir content to less than 50 at% ...while maintaining or even enhancing the overall electrocatalytic activity and durability remains a grand challenge. Herein, by dealloying predesigned Al‐based precursor alloys, it is possible to controllably incorporate Ir with another four metal elements into one single nanostructured phase with merely ≈20 at% Ir. The obtained nanoporous quinary alloys, i.e., nanoporous high‐entropy alloys (np‐HEAs) provide infinite possibilities for tuning alloy's electronic properties and maximizing catalytic activities owing to the endless element combinations. Particularly, a record‐high OER activity is found for a quinary AlNiCoIrMo np‐HEA. Forming HEAs also greatly enhances the structural and catalytic durability regardless of the alloy compositions. With the advantages of low Ir loading and high activity, these np‐HEA catalysts are very promising and suitable for activity tailoring/maximization.
To lower the Ir content and enhance the oxygen evolution reaction (OER) performance, a series of nanoporous high‐entropy alloys with ≈20 at% Ir are prepared by a dealloying method. Due to the multiprinciple element property and endless possibilities for electronic structure adjustment, a highly active AlNiCoIrMo catalyst is found for both OER and hydrogen evolution reaction (HER) in acidic media.
Developing bifunctional electrocatalysts with high activities and long durability for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial toward the practical ...implementation of rechargeable metal–air batteries. Here, a 3D nanoporous graphene (np‐graphene) doped with both N and Ni single atoms/clusters is reported. The predoping of N by chemical vapor deposition (CVD) dramatically increases the Ni doping amount and stability. The resulting N and Ni codoped np‐graphene has excellent electrocatalytic activities for both the ORR and the OER in alkaline aqueous solutions. The synergetic effects of N and Ni dopants are revealed by density functional theory calculations. The free‐standing Ni,N codoped 3D np‐graphene shows great potential as an economical catalyst/electrode for metal–air batteries.
The presence of N facilitates the loading of a high density of isolated Ni single atoms on graphene. The Ni and N codopod 3D nanoporous graphene exhibits greatly enhanced bifunctional oxygen catalytic activities due to the synergetic effect between Ni and N. Due to the superior catalytic performance and structure advantage, a nanoporous graphene‐based all‐solid‐state Zn–air battery exhibits superior performance.
One major challenge in heterogeneous catalysis is to reduce the usage of noble metals while maintaining the overall catalytic stability and efficiency in various chemical environments. In this work, ...a series of high‐entropy catalysts are synthesized by a chemical dealloying method and find the increased entropy effect and non‐noble metal contents would facilitate the formation of complete oxides with low crystallinity. Importantly, an optimal eight‐component high‐entropy oxide (HEO, Al‐Ni‐Co‐Ru‐Mo‐Cr‐Fe‐Ti) is identified, which exhibits further enhanced catalytic activity for the oxygen evolution reaction (OER) as compared to the previously reported quinary AlNiCoRuMo and the widely‐used commercial RuO2 catalysts, and at the same time similar catalytic activity for the oxygen reduction reaction (ORR) as the commercial Pt/C with a half‐wave potential of 0.87 V. Such high‐performance bi‐functional catalysts, however, only require a half loading amount of Ru as compared to the quinary AlNiCoRuMo, due to the underlying Cr‐Fe synergistic effects on tuning the electronic structures at active surface sites, as revealed by the first‐principles density functional theory calculations of the authors. The eight‐component HEO also demonstrates excellent stability under continuous electrochemical working conditions, suitable for a wide range of applications such as metal‐air batteries.
Owing to the synergistic effect of Cr+Fe, an eight‐component high‐entropy spinel oxide (AlNiCoRuMoCrFeTi) with a very low content of Ru and multiple active sites is found to be highly active for both the electrochemical oxygen evolution and reduction reactions. This work shows a route to design catalysts with low noble contents, tunable properties, and high stability.
The development of low‐Pt catalysts with high activity and durability is critical for fuel cells. Here, Pt‐skin wrapped sub‐5 nm PtCo intermetallic nanoparticles are successfully mounted on single ...atom Co‐N‐C support by exploiting the barrier effect of Co‐anchor. According to a collaborative experimental and computational investigation, the increased oxygen reduction reaction activity of PtCo/Co‐N‐C arises from the direct electron transfer from PtCo to Co‐N‐C, and the resulting optimal d‐band center of Pt. Owing to such unique electronic structure interaction and synergistic effect, the specific and mass activities of PtCo/Co‐N‐C are up to 4.20 mA cm−2 and 2.71 A mgPt−1, respectively, with barely degraded stability after 40 000 CV cycles. The PtCo/Co‐N‐C also exhibits outstanding activity as an ethanol electrocatalyst. This work shows a new and effective route to boost the overall efficiency of direct ethanol fuel cells in acidic media by integrating intermetallic low‐Pt alloys and single atom carbon support.
By exploiting the barrier effect of Co‐anchor, sub‐5 nm PtCo intermetallic nanoparticles are successfully mounted on single atom Co‐N‐C support. Owing to the unique structure and the electronic interaction between PtCo intermetallic compounds and Co‐N‐C, the PtCo/Co‐N‐C exhibits promising oxygen reduction reaction and ethanol oxidation reaction activities in acidic media and works well in the solid‐state direct ethanol fuel cell.
Starting from metal oxides, B4C and graphite, a suite of high-entropy boride ceramics, formulated (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2, (Hf0.2Zr0.2Mo0.2Nb0.2Ti0.2)B2 and (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 ...derived from boro/carbothermal reduction at 1600 °C were fabricated by spark plasma sintering at 2000 °C. It was found that the synthetic high-entropy boride crystalized in hexagonal structure and the yield of the targeting phase was calculated to be over 93.0 wt% in the sintered ceramics. Benefitting from the nearly full densification (96.3% ˜ 98.5% in relative density) and the refined microstructure, the products exhibited the relatively high Vickers hardness. The indentation fracture toughness was determined to be comparable with the single transition metal-diboride ceramics. It should be noted that the formation of high-entropy boride ceramics were featured with the relatively high hardness at no expense of the fracture toughness.
Developing single‐atomic catalysts with superior selectivity and outstanding stability for CO2 electroreduction is desperately required but still challenging. Herein, confinement strategy and ...three‐dimensional (3D) nanoporous structure design strategy are combined to construct unsaturated single Ni sites (Ni‐N3) stabilized by pyridinic N‐rich interconnected carbon nanosheets. The confinement agent chitosan and its strong interaction with g‐C3N4 nanosheet are effective for dispersing Ni and restraining their agglomeration during pyrolysis, resulting in ultrastable Ni single‐atom catalyst. Due to the confinement effect and structure advantage, such designed catalyst exhibits a nearly 100% selectivity and remarkable stability for CO2 electroreduction to CO, exceeding most reported state‐of‐the‐art catalysts. Specifically, the CO Faradaic efficiency (FECO) maintains above 90% over a broad potential range (‐0.55 to ‐0.95 V vs. RHE) and reaches a maximum value of 99.6% at a relatively low potential of ‐0.67 V. More importantly, the FECO is kept above 95% within a long‐term 100 h electrolyzing. Density functional theory (DFT) calculations explain the high selectivity for CO generation is due to the high energy barrier required for hydrogen evolution on the unsaturated Ni‐N3. This work provides a new designing strategy for the construction of ultrastable and highly selective single‐atom catalysts for efficient CO2 conversion.
Unsaturated single Ni sites (Ni‐N3) anchored on pyridinic N‐rich interconnected carbon nanosheets are constructed via confinement strategy. Due to the confinement effect and structure advantage, such designed catalyst exhibits a nearly 100% selectivity and remarkable stability (100 h) for CO2 electroreduction to CO.
As in many other electrochemical energy‐converting systems, the flexible direct ethanol fuel cells rely heavily on high‐performance catalysts with low noble metal contents and high tolerance to ...poisoning. In this work, a generic dealloying procedure to synthesize nanoporous multicomponent anodic and cathodic catalysts for the high‐performance ethanol fuel cells is reported. On the anode side, the nanoporous AlPdNiCuMo high‐entropy alloy exhibits an electrochemically active surface area of 88.53 m2 g−1Pd and a mass activity of 2.67 A mg−1Pd for the ethanol oxidation reaction. On the cathode side, the dealloyed spinel (AlMnCo)3O4 nanosheets with no noble metals demonstrate a comparable catalytic performance as the standard Pt/C for the oxygen reduction reaction, and tolerance to high concentrations of ethanol. Equipped with such anodic and cathodic catalysts, the flexible solid‐state ethanol fuel cell is able to deliver an ultra‐high energy density of 13.63 mWh cm−2 with only 3 mL ethanol, which is outstanding compared with other similar solid‐state energy devices. Moreover, the solid‐state ethanol fuel cell is highly flexible, durable and exhibits an inject‐and‐run function.
Flexible solid‐state direct ethanol fuel cells show great promise in various applications. In this work, nanoporous high‐entropy AlPdNiCuMo face‐centered cubic alloy and (AlMnCo)3O4 spinel oxides are prepared and used as anodic and cathodic catalysts to enhance fuel cell performance. A high energy density of 13.63 mWh cm−2, flexibility, and inject‐and‐run function are achieved.
•A model optimizing both quality and quantity of hydro/PV power was proposed.•The dimension was reduced by decoupling hydropower and PV power in time scales.•Reservoir operations have been optimized ...for different typical hydrological years.•Hydropower was proved to be an ideal compensating resource for PV power in nature.
The most striking feature of the solar energy is its intermittency and instability resulting from environmental influence. Hydropower can be an ideal choice to compensate photovoltaic (PV) power since it is easy to adjust and responds rapidly with low cost. This study proposed a long-term multi-objective optimization model for integrated hydro/PV power system considering the smoothness of power output process and the total amount of annual power generation of the system simultaneously. The PV power output is firstly calculated by hourly solar radiation and temperature data, which is then taken as the boundary condition for reservoir optimization. For hydropower, due to its great adjustable capability, a month is taken as the time step to balance the simulation cost. The problem dimension is thus reduced by decoupling hydropower and PV power in time scales. The modified version of Non-dominated Sorting Genetic Algorithm (NSGA-II) is adopted to optimize the multi-objective problem. The proposed model was applied to the Longyangxia hydro/PV hybrid power system in Qinghai province of China, which is supposed to be the largest hydro/PV hydropower station in the world. The results verified that the hydropower is an ideal compensation resource for the PV power in nature, especially in wet years, when the solar radiation decreases due to rainfalls while the water resource is abundant to be allocated. The power generation potential is provided for different hydrologic years, which can be taken to evaluate the actual operations. The proposed methodology is general in that it can be used for other hydro/PV power systems than those studied here.
