Multifunctionalization is the future development direction for microwave absorbing materials, but has not yet been explored. The effective integration of multiple functions into one material remains ...a huge challenge. Herein, an aerogel‐type microwave absorber assembled with multidimensional organic and inorganic components is synthesized. Polyacrylonitrile fibers and polybenzoxazine membranes work as the skeleton and crosslinker, respectively, forming a 3D framework, in which carbon nanotubes are interconnected into an electrically conductive network, and Fe3O4 nanoparticles are uniformly dispersed throughout the aerogel. Remarkably, the microwave absorption performances of the aerogel achieve ultralight, ultrathin (1.5 mm), and strong absorption (reflection loss of −59.85 dB) features. In particular, its specific reflection loss values considerably outperform the current magnetic–dielectric hybrids with similar components. Moreover, the aerogel possesses strong hydrophobicity and good thermal insulation, endowing it attractive functions of self‐cleaning, infrared stealth, and heat insulation that is even comparable to commercial products. The excellent multifunction benefits from the cellular structure of aerogel, the assembly of multidimensional nanomaterials, and the synergistic effect of organic–inorganic components. This study paves the way for designing next‐generation microwave absorbing materials with great potential for multifunctional applications.
A multifunctional microwave absorbing aerogel is designed and synthesized by assembling a variety of organic and inorganic components including polyacrylonitrile fibers, polybenzoxazine membranes, carbon nanotubes (CNTs), and Fe3O4 nanoparticles. This lightweight aerogel not only achieves excellent microwave absorption performances that outperform the current CNT‐based magnetic–dielectric hybrids, but also possesses self‐cleaning, infrared stealth, and thermal insulation functions.
The environmentally friendly synthesis of highly active Fe‐N‐C electrocatalysts for proton‐exchange membrane fuel cells (PEMFCs) is desirable but remains challenging. A simple and scalable method is ...presented to fabricate FeII‐doped ZIF‐8, which can be further pyrolyzed into Fe‐N‐C with 3 wt % of Fe exclusively in Fe‐N4 active moieties. Significantly, this Fe‐N‐C derived acidic PEMFC exhibits an unprecedented current density of 1.65 A cm−2 at 0.6 V and the highest power density of 1.14 W cm−2 compared with previously reported NPMCs. The excellent PEMFC performance can be attributed to the densely and atomically dispersed Fe‐N4 active moieties on the small and uniform catalyst nanoparticles.
An environmentally friendly, facile, low‐cost, and scalable solid‐phase synthesis was developed for a highly active Fe‐N‐C electrocatalyst with iron exclusively in Fe‐N4 moieties. The as‐prepared Fe2‐Z8‐C delivered an unprecedented power density of 1.14 W cm−2 in acidic proton‐exchange membrane fuel cell, which was attributed to the densely and atomically dispersed Fe‐N4 active moieties and the small catalyst nanoparticles.
M–N–C (M = Fe, Co) are highly active nonprecious metal electrocatalysts for the oxygen reduction reaction (ORR) and other applications. Although their operation stability has been extensively studied ...in proton‐exchange‐membrane fuel cells, the storage stability that determines the performance maintenance before use has not yet been understood. Here, it is found that long‐term exposure of M–N–C catalysts in air would cause surface oxidation and hydroxylation, resulting in significant decrease of ORR activity and fuel‐cell performances. Hydrogen passivation is demonstrated to be an effective strategy to protect the atomic M–N4 active sites and improve the storage stability of the catalysts. In addition, the hydrogen‐termination can also reduce the ORR energy barrier and increase the utilization of active sites, leading to the improvements of fuel‐cell activity and power density. Notably, these findings help to understand the storage‐associated degradation and protection of M–N–C catalysts.
The storage stability of M–N–C catalysts is investigated. Surface oxidation and hydroxylation are found to be responsible for serious performance degradation after long‐term storage in ambient environment. A hydrogen passivation strategy is proposed to modify and protect the catalytic active sites, which can greatly improve the storage stability as well as the fuel‐cell activity and power density of the catalysts.
Abstract
Single-atom catalysts (SACs) have sparked broad interest recently while the low metal loading poses a big challenge for further applications. Herein, a dual protection strategy has been ...developed to give high-content SACs by nanocasting SiO
2
into porphyrinic metal–organic frameworks (MOFs). The pyrolysis of SiO
2
@MOF composite affords single-atom Fe implanted N-doped porous carbon (Fe
SA
–N–C) with high Fe loading (3.46 wt%). The spatial isolation of Fe atoms centered in porphyrin linkers of MOF sets the first protective barrier to inhibit the Fe agglomeration during pyrolysis. The SiO
2
in MOF provides additional protection by creating thermally stable FeN
4
/SiO
2
interfaces. Thanks to the high-density Fe
SA
sites, Fe
SA
–N–C demonstrates excellent oxygen reduction performance in both alkaline and acidic medias. Meanwhile, Fe
SA
–N–C also exhibits encouraging performance in proton exchange membrane fuel cell, demonstrating great potential for practical application. More far-reaching, this work grants a general synthetic methodology toward high-content SACs (such as Fe
SA
, Co
SA
, Ni
SA
).
Hydrogen storage materials are the key to hydrogen energy utilization. However, current materials can hardly meet the storage capacity and/or operability requirements of practical applications. Here ...we report an advancement in hydrogen storage performance and related mechanism based on a hydrofluoric acid incompletely etched MXene, namely, a multilayered Ti2CTx (T is a functional group) stack that shows an unprecedented hydrogen uptake of 8.8 wt% at room temperature and 60 bar H2. Even under completely ambient conditions (25 °C, 1 bar air), Ti2CTx is still able to retain ~4 wt% hydrogen. The hydrogen storage is stable and reversible in the material, and the hydrogen release is controllable by pressure and temperature below 95 °C. The storage mechanism is deduced to be a nanopump-effect-assisted weak chemisorption in the sub-nanoscale interlayer space of the material. Such a storage approach provides a promising strategy for designing practical hydrogen storage materials.An incompletely etched Ti2CTx stack exhibits highly reversible hydrogen storage under near-ambient conditions by nanopump-effect-assisted weak chemisorption.
Replacing precious platinum with earth‐abundant materials for the oxygen reduction reaction (ORR) in fuel cells has been the objective worldwide for several decades. In the last 10 years, the ...fastest‐growing branch in this area has been carbon‐based metal‐free ORR electrocatalysts. Great progress has been made in promoting the performance and understanding the underlying fundamentals. Here, a comprehensive review of this field is presented by emphasizing the emerging issues including the predictive design and controllable construction of porous structures and doping configurations, mechanistic understanding from the model catalysts, integrated experimental and theoretical studies, and performance evaluation in full cells. Centering on these topics, the most up‐to‐date results are presented, along with remarks and perspectives for the future development of carbon‐based metal‐free ORR electrocatalysts.
Replacing precious platinum with earth‐abundant materials in fuel cells for the oxygen reduction reaction (ORR) has been a worldwide objective for several decades. In the last 10 years, the fastest‐growing branch in this area has been carbon‐based metal‐free ORR electrocatalysts. The most up‐to‐date results of this field are presented, along with remarks and perspectives for the future development of carbon‐based metal‐free ORR electrocatalysts.
Abstract
Improving the accessibility of ions in the electrodes of electrochemical energy storage devices is vital for charge storage and rate performance. In particular, the kinetics of ion transport ...in organic electrolytes is slow, especially at low operating temperatures. Herein, we report a new type of MXene-carbon nanotube (CNT) composite electrode that maximizes ion accessibility resulting in exceptional rate performance at low temperatures. The improved ion transport at low temperatures is made possible by breaking the conventional horizontal alignment of the two-dimensional layers of the MXene Ti
3
C
2
by using specially designed knotted CNTs. The large, knot-like structures in the knotted CNTs prevent the usual restacking of the Ti
3
C
2
flakes and create fast ion transport pathways. The MXene-knotted CNT composite electrodes achieve high capacitance (up to 130 F g
−1
(276 F cm
−3
)) in organic electrolytes with high capacitance retention over a wide scan rate range of 10 mV s
−1
to 10 V s
−1
. This study is also the first report utilizing MXene-based supercapacitors at low temperatures (down to −60 °C).
Porous carbon nanotubes/cobalt nanoparticles (CNTs/Co) composite with dodecahedron morphology was synthesized by in situ pyrolysis of the Co-based zeolitic imidazolate framework in a reducing ...atmosphere. The morphology and microstructure of the composite can be well tuned by controlling the pyrolysis conditions. At lower pyrolysis temperature, the CNTs/Co composite is composed of well-dispersed Co nanoparticles and short CNT clusters with low graphitic degree. The increase of pyrolysis temperature/time promotes the growth and graphitization of CNTs and leads to the aggregation of Co nanoparticles. The optimized CNTs/Co composite exhibits strong dielectric and magnetic losses as well as a good impedance matching property. Interestingly, the CNTs/Co composite displays extremely strong electromagnetic wave absorption with a maximum reflection loss of −60.4 dB. More importantly, the matching thickness of the absorber is as thin as 1.81 mm, and the filler loading of composite in the matrix is only 20 wt %. The highly efficient absorption is closely related to the well-designed structure and the synergistic effect between CNTs and Co nanoparticles. The excellent absorbing performance together with lightweight and ultrathin thickness endows the CNTs/Co composite with the potential for application in the electromagnetic wave absorbing field.
Fe−N−C catalysts with single‐atom Fe−N4 configurations are highly needed owing to the high activity for oxygen reduction reaction (ORR). However, the limited intrinsic activity and dissatisfactory ...durability have significantly restrained the practical application of proton‐exchange membrane fuel cells (PEMFCs). Here, we demonstrate that constructing adjacent metal atomic clusters (ACs) is effective in boosting the ORR performance and stability of Fe−N4 catalysts. The integration of Fe−N4 configurations with highly uniform Co4 ACs on the N‐doped carbon substrate (Co4@/Fe1@NC) is realized through a “pre‐constrained” strategy using Co4 molecular clusters and Fe(acac)3 implanted carbon precursors. The as‐developed Co4@/Fe1@NC catalyst exhibits excellent ORR activity with a half‐wave potential (E1/2) of 0.835 V vs. RHE in acidic media and a high peak power density of 840 mW cm−2 in a H2−O2 fuel cell test. First‐principles calculations further clarify the ORR catalytic mechanism on the identified Fe−N4 that modified with Co4 ACs. This work provides a viable strategy for precisely establishing atomically dispersed polymetallic centers catalysts for efficient energy‐related catalysis.
Constructing metal atomic clusters can effectively boost the oxygen reduction reaction (ORR) performance and stability of Fe−N4 on the N‐doped carbon (NC) substrate. The Co4@/Fe1@NC catalyst is realized through a “pre‐constrained” strategy using Co4 molecular clusters and Fe(acac)3 implanted carbon precursors and the obtained catalyst exhibits excellent ORR activity.
Simultaneously increasing the activity and stability of the single-atom active sites of M-N-C catalysts is critical but remains a great challenge. Here, we report an Fe-N-C catalyst with ...nitrogen-coordinated iron clusters and closely surrounding Fe-N
active sites for oxygen reduction reaction in acidic fuel cells. A strong electronic interaction is built between iron clusters and satellite Fe-N
due to unblocked electron transfer pathways and very short interacting distances. The iron clusters optimize the adsorption strength of oxygen reduction intermediates on Fe-N
and also shorten the bond amplitude of Fe-N
with incoherent vibrations. As a result, both the activity and stability of Fe-N
sites are increased by about 60% in terms of turnover frequency and demetalation resistance. This work shows the great potential of strong electronic interactions between multiphase metal species for improvements of single-atom catalysts.
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