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1.
ChemSusChem ; 2020 Mar 02.
Artigo em Inglês | MEDLINE | ID: mdl-32119757

RESUMO

Recent efforts to improve the capacitances of Kraft lignin (KL) in supercapacitors have mainly focused on screening KL substrates, which could either compensate the poor conductivity of KL or directly contribute to the capacitance. However, increasing the pseudocapacitance contributed by KL itself, through hydroquinone/quinone redox cycles, remains a challenge, owing to the roughly fixed content of hydroquinone species in natural KL. In this study, the capacitance of KL is greatly improved by using a functional-group modification strategy in which methoxy groups in KL are selectively converted into phenolic hydroxy groups, which facilitate the formation of additional hydroquinone moieties and thus lead to higher pseudocapacitances. The oxidized KL materials show up to 25.6 % enhancement of the phenolic hydroxy content in comparison to raw KL, which results in 21.9 % capacitance improvement from 322 to 390 F g-1 at 0.5 A g-1 in an acidic system.

2.
Chem Commun (Camb) ; 2020 Jan 29.
Artigo em Inglês | MEDLINE | ID: mdl-31994548

RESUMO

The rational manipulation of reaction intermediates is crucial for achieving high-performance heterogeneous catalysis. Herein, using in situ Fourier transform infrared-diffuse reflection (FTIR) analysis, we report that the methanol oxidation reaction (MOR) intermediates can be controlled by precisely tuning the location and content of Ru on the Pt-Ru alloy surface.

3.
Artigo em Inglês | MEDLINE | ID: mdl-31831406

RESUMO

OBJECTIVE: The purpose of this work was to build a radio frequency (RF) coil system to achieve high vessel wall image quality with coverage extending from the aortic arch to the intracranial vessels. METHODS: A 48-channel coil system was built and characterized at a 3 tesla (T) Magnetic Resonance Imaging (MRI) scanner (uMR 790, Shanghai United Imaging Healthcare, Shanghai, China). The coil's performance was compared with a commercially available 36-channel coil system. By human studies, signal-to-noise ratio (SNR) units were evaluated and g-factors were calculated in the transverse planes of the brain and neck regions. RESULTS: The SNR was increased by at least 28% in the brain region and up to fourfold in the neck region. The average g-factor with the acceleration factor, R=3, was lowered by 21% in the transverse plane of the neck region. Intracranial and carotid arterial wall images with an isotropic spatial resolution of 0.63 mm were acquired within 7.7 minutes and thoracic aorta wall images with an isotropic spatial resolution of 1.1 mm were acquired within 2.7 minutes with the 48-channel coil system. The vessel wall can be more clearly visualized with the 48-channel coil system compared with the 36-channel coil system. CONCLUSION: A 48-channel coil system was developed and demonstrated superior performance for vessel wall imaging at the intracranial and cervical carotid arteries compared with a commercial 36-channel coil. SIGNIFICANCE: The 48-channel coil system is potentially useful for clinical diagnostics, especially when attempting to diagnose ischemic stroke.

4.
Chemistry ; 2019 Oct 25.
Artigo em Inglês | MEDLINE | ID: mdl-31769561

RESUMO

The large-scale application of polymer electrolyte membrane fuel cells (PEMFCs) depends heavily on the development of high-performance and cost-effective oxygen reduction reaction (ORR) electrocatalysts to replace the traditional Pt-based materials. Among the numerous candidates of ORR electrocatalysts, precious metal-free carbon-based materials have received ever-increasing attention for their unique electronic features, tunable nanostructures, and robustness. Although tremendous efforts have been devoted to raising the intrinsic properties of precious metal-free carbon-based materials, there exists a lot of room for them to be improved in activity, stability, and poison-tolerance. In this Minireview, the latest strategies for enhancing activity by increasing the accessible active sites and promoting the intrinsic activity have been summarized. In addition, with special emphasis on devastating catalyst poisoning, up-to-data tactics for elevating the stability and poison-tolerance are introduced. Finally, conclusions and perspectives are also presented.

5.
Chem Commun (Camb) ; 55(96): 14482-14485, 2019 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-31729511

RESUMO

The electrochemical nitrogen reduction reaction (NRR) is a promising but extremely challenging approach for ammonia synthesis under ambient conditions. Herein, we report the excellent NRR performance of gold nanoparticles (AuNPs) with multiple high-index facets, prepared by a modified seed-mediated method. At -0.3 V vs. RHE and in 0.1 M Li2SO4 aqueous solution, the AuNPs afford the highest faradaic efficiency (FE) of 73.32% reported so far, with a remarkable ammonia generation rate of 9.22 µg h-1 cm-2. Density functional theory (DFT) calculations reveal that the high-index faceted surfaces of the AuNPs have greater preference for the adsorption of NRR intermediates (*NNH) and significantly hinder the adsorption of competing hydrogen evolution intermediates (*H).

6.
Phys Chem Chem Phys ; 21(47): 26102-26110, 2019 Dec 04.
Artigo em Inglês | MEDLINE | ID: mdl-31748776

RESUMO

To understand the essential reasons of poor durability and rapid initial performance loss of heteroatom doped graphene catalysts during the electrochemical oxygen reduction reaction (ORR) process, it is necessary to explore the detailed mechanism of carbon active site oxidation reaction (COR) at different electrode potentials, as it may greatly influence the ORR activity. Herein, density functional theory (DFT) calculation is used to investigate all possible COR mechanisms, including Direct-COR and Indirect-COR, on four typical doped-graphene, and understand the competing relation between COR and ORR from a thermodynamic point of view. Our systematic calculations found that the Direct-COR is affected directly by the structural stability of doped-graphene relative to pure graphite, and the Indirect-COR can be accelerated largely by the ORR process due to the ORR intermediate, such as O and OOH. The competition relation between COR and ORR is mainly influenced by the interaction between the doped-graphene and reaction species, stability of doped-structure, ORR mechanism, and electrode potential. For COR, the partial oxidation of doped-graphene is the dominant oxidation reaction compared to complete oxidation in the ORR potential range. More importantly, both partial and complete oxidation of doped-graphene can remarkably depress the ORR activity. Hence, COR should be one of the major contributors to the rapid initial performance loss of carbon based catalysts in stability testing. Our results provide a comprehensive and deep understanding of the oxidation of carbon active sites on doped-graphene surfaces and can guide the design of more robust doped-carbon based catalysts.

7.
Nanoscale ; 11(42): 20115-20122, 2019 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-31612897

RESUMO

Carbon supported Pt-based alloy materials that have been developed for proton exchange membrane fuel cells (PEMFCs) are vulnerable to deactivation due to the loss of non-noble metal components (leaching) or detachment, migration and aggregation of active nanoparticles (sintering). Until now some methods have been developed to inhibit leaching or sintering individually. However, a route able to avoid leaching and sintering simultaneously is still lacking. Herein, we develop a thermally driven interfacial diffusion alloying route that allows for the direct evolution of solid Pt nanoparticles (NPs) supported on carbon (Pt/C) into a Pt-skin-like hollow PtFe alloy or a structurally ordered intermetallic PtFe alloy, together with in situ encapsulation of PtFe alloy NPs with a thin layer porous nitrogen-doped carbon (NC) shell. The robust NC shells not only effectively prevent Pt-based NPs from detachment, migration, and aggregation during accelerated durability tests but also allow smoother access of electrolyte to the Pt surface, thus allowing the catalysts to well preserve their high catalytic activity. The well-defined shape and atomic arrangement of PtFe alloy NPs exhibit over 600% increase in mass activity and specific activity when compared with that of the pristine Pt/C catalyst. Stability tests confirm that the ordered PtFe alloy is more electrochemically stable than the disordered hollow PtFe alloy and Pt/C catalysts due to its ordered atomic arrangement and the robust NC shell.

8.
Chem Commun (Camb) ; 55(80): 12028-12031, 2019 Oct 03.
Artigo em Inglês | MEDLINE | ID: mdl-31531451

RESUMO

Herein, we present a high-temperature self-assembly strategy that directly allows the transformation of adsorbed Pt(NH3)42+ and Fe3+ sources into structurally ordered face-centered tetragonal (fct)-PtFe alloy NPs (2.6 ± 0.2 nm) by integrating reduction and phase transformation. The small-size and ordered atomic arrangement of the fct-PtFe alloy NPs, together with their robust NC protective shell, make these NPs exhibit excellent catalytic activity and stability for the oxygen reduction reaction.

9.
Chem Commun (Camb) ; 55(61): 9023-9026, 2019 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-31290864

RESUMO

A high-performance 3D hierarchical porous metal-free N-doped carbon catalyst toward the oxygen reduction reaction (ORR) in acidic medium was successfully synthesized by employing ZnO as a mesoporous template and NaCl as both a macroporous template and a structure protective agent. The resultant improved active site density and diffusion efficiency lead to a superior ORR activity with a half-wave potential high up to 0.755 V in 0.1 M HClO4.

10.
ACS Appl Mater Interfaces ; 11(25): 22290-22296, 2019 Jun 26.
Artigo em Inglês | MEDLINE | ID: mdl-31150203

RESUMO

Herein, we report an efficient strategy to transfer metal organic frameworks into huge-diameter carbon nanotubes (CNTs) at high production by using Fe-citrate-functionalized zeolitic imidazolium frameworks-8 (ZIF-8) as precursors. The constructed porous Fe-N-decorated CNTs represent a tube diameter of >0.1 µm and a length of >5 µm. We find that the use of Fe-citrate could not only facilitate the agglomeration of Fe to generate huge-diameter CNTs by the bottom-up organic strategy but also induce the formation of Fe-N sites by the ligand substitution of 2-methylimidazole with citrate ions in ZIF-8, making it a promising class of Pt-alternative catalysts for oxygen reduction reaction (ORR). Such an ideal architecture is endowed with highly intrinsic activity from Fe-N and the faster mass transport from huge-diameter tubes, leading to an excellent ORR performance with a maximum power density of 665 mW cm-2.

11.
Phys Chem Chem Phys ; 21(24): 12826-12836, 2019 Jun 28.
Artigo em Inglês | MEDLINE | ID: mdl-31165824

RESUMO

A single-atom TM-Nx (TM = Fe, Co, Mn, etc.) embedded graphene matrix is known for its excellent activity and durability in oxygen reduction reaction (ORR) catalysis. Among them, Mn-N4 sites have been theoretically proved to undergo a complete 4-electron pathway with low ORR overpotentials and low activation barriers in O2 dissociation. However, in reality there still remain significant activity gaps between such Mn-N4 based catalysts (such as MnPc and MnP) and Fe-N4 or Pt-group metal catalysts. The inferior ORR performance of MnPc and MnP could be attributed to the strong binding ability of Mn that causes great difficulties in removing the ORR products from the surface sites. On this basis, 17 types of Mn-Nx models containing various three-, four- and five-coordination groups were established. Systematic density functional theory (DFT) calculations were performed to investigate the N,C coordination effects on their corresponding ORR activities. Scaling relations were found among the binding strengths of key ORR intermediates, which could be modulated by the N doping level among different coordination groups. A volcano plot for ORR overpotentials (ηSHE) as a function of *OH adsorption free energy (ΔG*OH) was further established. The 3D five-coordination sites exhibit much higher ORR activity due to the great decrease in strong binding abilities compared with 2D three- or four-coordination sites. Particularly, (Cyan)Mn-N4/D is positioned near the apex of the volcano plot with an ηSHE of 0.33 V even lower than that of Pt(111) (0.34 V). Furthermore, the electron withdrawing/donating mechanisms among Mn, N, C, and O were investigated and related to the binding abilities of different coordination groups. Electronic structure calculations indicate that the binding abilities of Mn-Nx well correlate with the σ-type anti-bonding components between Mn-3d and O-2p states near the Fermi energy level.

12.
Chemistry ; 25(42): 9799-9815, 2019 Jul 25.
Artigo em Inglês | MEDLINE | ID: mdl-30938875

RESUMO

Interface modulation, as an old concept of heterogeneous catalysis, represents an emerging, fast-growing and exciting direction in the field of water electrolysis. Over the past five years, diverse hetero-nanostructures have been synthesised as water electrolysis catalysts by taking advantage of interface modulation. However, it seems that the performance (i.e., efficiency and durability) of these materials needs to be further improved. Therefore, a comprehensive summary of recent achievements and the challenging issues concerning the regulation of material functionalities through interface modulation is necessary and helpful. Herein, firstly, the fundamentals of water electrolysis are outlined, and then the delicate design and fine control of well-defined interfaces, as well as related mechanisms for performance improvement are discussed. Finally, future opportunities and challenges in the everlasting pursuit of highly efficient and robust water electrolysis catalysts are highlighted.

13.
Angew Chem Int Ed Engl ; 58(21): 7035-7039, 2019 May 20.
Artigo em Inglês | MEDLINE | ID: mdl-30895689

RESUMO

Atomically dispersed Zn-N-C nanomaterials are promising platinum-free catalysts for the oxygen reduction reaction (ORR). However, the fabrication of Zn-N-C catalysts with a high Zn loading remains a formidable challenge owing to the high volatility of the Zn precursor during high-temperature annealing. Herein, we report that an atomically dispersed Zn-N-C catalyst with an ultrahigh Zn loading of 9.33 wt % could be successfully prepared by simply adopting a very low annealing rate of 1° min-1 . The Zn-N-C catalyst exhibited comparable ORR activity to that of Fe-N-C catalysts, and significantly better ORR stability than Fe-N-C catalysts in both acidic and alkaline media. Further experiments and DFT calculations demonstrated that the Zn-N-C catalyst was less susceptible to protonation than the corresponding Fe-N-C catalyst in an acidic medium. DFT calculations revealed that the Zn-N4 structure is more electrochemically stable than the Fe-N4 structure during the ORR process.

14.
Phys Chem Chem Phys ; 21(9): 4899-4906, 2019 Feb 27.
Artigo em Inglês | MEDLINE | ID: mdl-30756098

RESUMO

Doping heteroatoms into phosphorene has attracted considerable attention due to its effectiveness in enhancing the stability and tailoring the electronic properties as well as sensing capability. In this work, we performed a density functional theory (DFT) calculation to study the effect of nonmetallic heteroatom dopants (X, X = Be, B, C, N, O, S, Se, Te, F, Cl, Br and I) on the stability and sensibility of X-doped phosphorene. We found that phosphorene structures doped with the 2nd period and VIA group atoms possess relatively high internal stability due to the effective X-P bonding. More interestingly, the interaction between P sites and small species can be modified effectively by a local property-the charge (Q) of P sites-and/or a global property-the Fermi level (EFermi) of X-doped phosphorene-depending on the charge transfer degree between the species and the P sites. For the species with dramatic charge transfer, the local Q of the P site plays the most important role. For the species with moderate charge transfer, the major determinant of sensibility is the local Q and global EFermi. This work reveals the relation between the properties of X-doped phosphorene and the physico-chemical parameters of X, including the electronegativity, atomic number, and X-P bonding strength, and provides a useful guideline for choosing the appropriate nonmetallic dopants to enhance the stability and sensibility of X-doped phosphorene towards special species.

15.
Phys Chem Chem Phys ; 21(6): 3242-3249, 2019 Feb 06.
Artigo em Inglês | MEDLINE | ID: mdl-30681699

RESUMO

Surface strain has been widely applied in catalyst design. It has been reported that tensile strain can weaken the adsorption of species on certain metal surfaces similar to the effects of compressive strain. This result contradicts the widely accepted rule predicated on the d-band center. Here, by using DFT calculations, we confirmed the abnormal adsorption behaviour of certain species on strained Pt low-index surfaces and found that the behaviour is dependent on the surfaces and species. Tensile strain on the close-packed Pt(111) and Pt(100) surfaces enhances species adsorption, while tensile strain on the open Pt(110) surface weakens species adsorption. This result is attributed to the asynchronous change in the five 5d orbitals due to the inconsistency between interlayer contraction and biaxial stretching. The dramatic contraction of interlayer spacing on the tensile strained Pt(110) surface sharply downshifts the dz2 center, then weakens species adsorption. Thus, due to the different roles of the five d orbitals in binding species, the inconsistent change in the five d orbitals is the intrinsic mechanism of the effects of strain on metal catalysts. Selectively tuning the five d orbitals might provide a new strategy to modify the adsorption behaviour of species on Pt-based catalysts and may result in extraordinarily high catalytic activities.

16.
Top Curr Chem (Cham) ; 377(1): 4, 2019 Jan 08.
Artigo em Inglês | MEDLINE | ID: mdl-30617518

RESUMO

The use of dispersive single-atom metals anchored on functionalized carbon nanomaterials as electrocatalysts for electrochemical energy conversion reactions represents a burgeoning area of research, due to their unique characteristics of low coordination number, uniform coordination environment, and maximum atomic utilization. Here we highlight the advanced synthetic methods, characterization techniques, and electrochemical applications for carbon-based single-atom metal catalysts, and provide illustrative correlations between molecular/electronic structures and specific catalytic activity for O2 reduction, water splitting, and other emerging reactions including CO2 reduction, H2O2 production, and N2 reduction. We also discuss fundamental principles for the future design of carbon-based single-atom metal catalysts for specific electrochemical reactions. In addition, we explore the challenges and opportunities that lie ahead in further work with carbon-based single-atom metal electrocatalysts.


Assuntos
Carbono/química , Técnicas Eletroquímicas , Metais/química , Nanopartículas/química
17.
ACS Appl Mater Interfaces ; 11(1): 699-705, 2019 Jan 09.
Artigo em Inglês | MEDLINE | ID: mdl-30543400

RESUMO

The electro-catalyzed oxygen reduction and evolution reactions (ORR/OER) are the key elements of many energy conversion systems, such as fuel cells, water electrolyzers, and rechargeable metal-air batteries. Structural design of durable non-noble nanomaterials as bifunctional OER/ORR catalysts is a major drawback to commercial applications. Herein, we exposed a strongly coupled hybrid material comprising of NiFeP-cubes nanoparticles supported on three-dimensional interconnected Fe,N-decorated carbon (3D-FeNC) as a robust bifunctional ORR/OER catalyst. The strongly coupled NiFeP@3D-FeNC catalyst shows better electron and mass transfer capability, exposure of abundant ORR/OER active sites on the surface, and strongly coupled effects. Accordingly, the as-prepared NiFeP@3D-FeNC catalyst exhibits robust ORR activity (half-wave potential of 0.84 V vs reversible hydrogen electrode) and OER performance (over-potential 0.25 V@10 mA cm-2) in alkaline media. Significantly, the oxygen electrode prepared from the NiFeP@3D-FeNC catalyst further demonstrated superior charge/discharge behavior and long-lasting rechargeability than the benchmark Pt/C + IrO2 catalyst in rechargeable zinc-O2 batteries. This approach opens up a new avenue for the synthesis and advanced the hybrid nanomaterials for various applications.

18.
Small ; 14(52): e1804277, 2018 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-30475459

RESUMO

Rechargeable lithium-sulfur batteries, which use sulfur as the cathode material, promise great potentials to be the next-generation high-energy system. However, higher-order lithium polysulfides, Li2 Sx (x = 4, 6, and 8), regardless of in charge or in discharge, always form first, dissolve subsequently in the electrolyte, and shuttle to the cathode and the anode, which is called "shuttle effect." The polysulfides shuttle effect leads to heavy loss of the active-sulfur materials. Literature works mostly "cover or fill" the pores to block polysulfides from shuttling, which also hinder the lithium ion transfer. Here a protocol is invented to grasp polysulfides based on the "soft and hard acid-base" theory. Tertiary amine layer (TAL) polymerized on a polypropylene separator selectively coordinates with the dissolved high-order Li2 Sx in the cathode. Meanwhile, the transportation of lithium cations is not interrupted because of enough pores left for their transportation. After 400 cycles of charge/discharge at 0.5C, the TAL modified separator battery still possesses a capacity of 865 mAh g-1 , which is among the best of the state-of-the-art performances of lithium-sulfur batteries. The flexible "polysulfides tongs" construction method paves a new way for Li-S batteries to reach desired performances with less worry about polysulfides shuttle.

19.
Small ; 14(52): e1804183, 2018 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-30457702

RESUMO

Hollow structured materials are widely applicable in various fields. Although many routes have been explored for getting such materials, a strategy mainly based on physical effect is still deficient. Herein, a "stresses induced orientation contraction" mechanism for preparation of hollow structures is reported. The composites constructed by zeolite imidazolate framework-8 (ZIF8) cores and polymerized dopamine (PDA) shells, upon annealing, form intensive interfacial interactions, which drag the ZIF8 cores outward to restrain their shrinkage. The gradually accumulated stresses in the central position of ZIF8 dodecahedron nanoparticles, then destroy the ZIF8 crystalline cores to form the hollow structures. In this stress-based route for creating hollow interiors with core-shell composites as the starting materials, three critical factors are necessary: 1) an intensive core-shell interfacial interaction; 2) the distinctly higher shrinkage degree of the cores than the shells; and 3) the relatively loose core structures. In oxygen reduction reaction (ORR) tested with three-electrode solution system and Zn-O2 battery, the achieved hollow nitrogen doped carbon (NC) demonstrates ultrahigh catalytic activities. This work gives an absolutely novel strategy for preparation of hollow structures, which may afford the exploration of a wider range of materials system with hollow interiors.

20.
Angew Chem Int Ed Engl ; 57(46): 15101-15106, 2018 Nov 12.
Artigo em Inglês | MEDLINE | ID: mdl-30203428

RESUMO

Electrocatalysts are readily poisoned during the catalysis of the oxygen reduction reaction (ORR); even air containing traces of SOx and/or NOx significantly decrease the activity and the durability of H2 -O2 fuel cells. Now, a metal-free strategy is reported to develop an efficient anti-poisoning ORR catalyst, which involves the pyrolysis of PDAP-phytic acid super-molecular aggregate (PPSA). The pyrolyzed co-doped carbon acting as a metal-free electrocatalyst shows an enhanced activity for ORR in acidic medium, even under poisoning conditions (SOx , NOx , and POx ). Moreover, P-doping also changes the ORR pathway by yielding less than 4 % of H2 O2 , indicating a four-electron pathway whereas more than>20 % of H2 O2 was recorded for N-doped carbon synthesized from PDAP.

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