Hierarchical sulfur and nitrogen co-doped carbon nanocages as efficient bifunctional oxygen electrocatalysts for rechargeable Zn-air battery

Exploring inexpensive and efficient bifunctional electrocatalysts for oxygen reduction reaction(ORR) and oxygen evolution reaction(OER) is critical for rechargeable metal-air batteries. Herein, we report a new 3D hierarchical sulfur and nitrogen co-doped carbon nanocages(hSNCNC) as a promising bifunctional oxygen electrocatalyst by an in-situ MgO template method with pyridine and thiophene as the mixed precursor. The as-prepared h SNCNC exhibits a positive half-wave potential of 0.792 V(vs. reversible hydrogen electrode, RHE) for ORR, and a low operating potential of 1.640 V at a 10 mA cm-2 current density for OER. The reversible oxygen electrode index is 0.847 V, far superior to commercial Pt/C and IrO2,which reaches the top level of the reported bifunctional catalysts. Consequently, the hSNCNC as air cathodes in an assembled Zn-air battery features low charge/discharge overpotential and long lifetime. The remarkable properties arises from the introduced multiple heteroatom dopants and stable 3D hierarchical structure with multi-scale pores, which provides the abundant uniform high-active S and N species and efficient charge transfer as well as mass transportation. These results demonstrate the potential strategy in developing suitable carbon-based bi-/multi-functional catalysts to enable the next generation of the rechargeable metal-air batteries.

Micro–meso-macroporous FeCo-N-C derived from hierarchical bimetallic FeCo-ZIFs as cathode catalysts for enhanced Li-O2 batteries performance

Developing bifunctional catalysts that increase both the OER and ORR kinetics and transport reactants with high efficiency is desirable. Herein, micro–meso-macroporous FeCo-N-C-X(denoted as "MFeCo-N-C-X", X represents Fe/Co molar ratio in bimetallic zeolite imidazole frameworks FeCo-ZIFs) catalysts derived from hierarchical M-FeCo-ZIFs-X was prepared. The micropores in M-FeCo-N-C-X have strong capability in O2 capture as well as dictate the nucleation and early-stage deposition of Li2O2,the mesopores provided a channel for the electrolyte wetting, and the macroporous structure promoted more available active sites when used as cathode for Li-O2 batteries. More importantly, M-Fe CoN-C-0.2 based cathode showed a high initial capacity(18,750 mAh g-1@0.1 A g-1), good rate capability(7900 m Ah g-1@0.5 A g-1), and cycle stability up to 192 cycles. Interestingly, the FeCo-N-C-0.2 without macropores suffered relatively poorer stability with only 75 cycles, although its discharge capacity was still as high as 17,200 mA h g-1(@0.1 A g-1). The excellent performance attributed to the synergistic contribution of homogeneous Fe, Co nanoparticles and N co-doping carbon frameworks with special micro–meso-macroporous structure. The results showed that hierarchical FeCo-N-C architectures are promising cathode catalysts for Li-O2 batteries.

Alloyed Pt-Sn nanoparticles on hierarchical nitrogen-doped carbon nanocages for advanced glycerol electrooxidation

Glycerol is an alternative sustainable fuel for fuel cells,and efficient electrocatalyst is crucial for glycerol oxidation reaction(GOR).The promising Pt catalysts are subject to the inadequate capability of C-C bond cleavage and the susceptibility to poisoning.Herein,Pt-Sn alloyed nanoparticles are immobilized on hierarchical nitrogen-doped carbon nanocages(hNCNCs)by convenient ethylene glycol reduction and subsequent thermal reduction.The optimal Pt_(3)Sn/hNCNC catalyst exhibits excellent GOR performance with a high mass activity(5.9 A·mg_(Pt)^(-1)),which is 2.7 and 5.4 times higher than that of Pt/hNCNC and commercial Pt/C,respectively.Such an enhancement can be mainly ascribed to the increased anti-poisoning and C-C bond cleavage capability due to the Pt_(3)Sn alloying effect and Sn-enriched surface,the high dispersion of Pt_(3)Sn active species due to N-participation,as well as the high accessibility of Pt_(3)Sn active species due to the three-dimensional(3D)hierarchical architecture of hNCNC.This study provides an effective GOR electrocatalyst and convenient approach for catalyst preparation.

Impact of Pd single-site coordination structure on catalytic performance for semihydrogenation of acetylene

Semihydrogenation of trace acetylene in an ethylene gas stream is a vital step for the industrial production of polyethylene,in which Pd single-site catalysts(SSCs)have great potential.Herein,two Pd SSCs with different coordination structures are prepared on hierarchical nitrogen-doped carbon nanocages(hNCNC)by regulating the nitrogen species with or without using dicyandiamide.With using dicyandiamide,the obtained Pd1-Ndicy/hNCNC SSC features the coordinated Pd by two pyridinic N and two pyrrolic N(PdN^(py)_(2)N^(pr)_(2)).Without using dicyandiamide,the obtained Pd1/hNCNC SSC features the coordinated Pd by pyridinic N and C(PdN^(py_(x)C_(4-x)),x=1-4).The former exhibits an 18-fold increase in catalytic activity compared to the latter.Theoretical results reveal the abundant unoccupied orbital states above the Fermi level of moiety,which can facilitate the activation of substrate molecules and dynamics of acetylene hydrogenation as supported by the combined theoretical and experimental results.In addition,PdN^(py)_(2)N^(pr)_(2)the moiety presents a favorable desorption of ethylene.Consequently,the Pd1-Ndicy/hNCNC SSC exhibits high C2H2 conversion(99%)and C2H4 selectivity(87%)at 160℃.This study demonstrates the impact of Pd single-site coordination structure on catalytic performance,which is significant for the rational design of advanced Pd SSCs on carbon-based supports.

Self-supported NiFe-LDH nanosheets on NiMo-based nanorods as high-performance bifunctional electrocatalysts for overall water splitting at industrial-level current densities

Efficient,durable and economic electrocatalysts are crucial for commercializing water electrolysis technology.Herein,we report an advanced bifunctional electrocatalyst for alkaline water splitting by growing NiFe-layered double hydroxide(NiFe-LDH)nanosheet arrays on the conductive NiMo-based nanorods deposited on Ni foam to form a three-dimensional(3D)architecture,which exhibits exceptional performances for both hydrogen evolution reaction(HER)and oxygen evolution reaction(OER).In overall water splitting,only the low operation voltages of 1.45/1.61 V are required to reach the current density of 10/500 mA·cm^(-2),and the continuous water splitting at an industrial-level current density of 500 mA·cm^(-2) shows a negligible degradation(1.8%)of the cell voltage over 1000 h.The outstanding performance is ascribed to the synergism of the HER-active NiMo-based nanorods and the OER-active NiFe-LDH nanosheet arrays of the hybridized 3D architecture.Specifically,the dense NiFe-LDH nanosheet arrays enhance the local pH on cathode by retarding OH-diffusion and enlarge the electrochemically active surface area on anode,while the conductive NiMo-based nanorods on Ni foam much decrease the charge-transfer resistances of both electrodes.This study provides an efficient strategy to explore advanced bifunctional electrocatalysts for overall water splitting by rationally hybridizing HER-and OER-active components.

Ultrasmall high-entropy alloy nanoparticles on hierarchical N-doped carbon nanocages for tremendous electrocatalytic hydrogen evolution

High-entropy alloys (HEAs) are promising candidates for the electrocatalyst of hydrogen evolution reaction (HER) due to their unique properties such as cocktail electronic effect and lattice distortion effect. Herein, the ultrasmall (sub-2 nm) nanoparticles of PtRuCoNiCu HEA with uniform element distribution are highly dispersed on hierarchical N-doped carbon nanocages (hNCNC) via low-temperature thermal reduction, denoted as us-HEA/hNCNC. The optimal us-HEA/hNCNC exhibits excellent HER performance in 0.5 M H_(2)SO4 solution, achieving an ultralow overpotential of 19 mV at 10 mA·cm^(−2) (without iR-compensation), high mass activity of 13.1 A·mgnoble metals ^(−1) at −0.10 V and superb stability with a slight overpotential increase of 3 mV after 20,000 cycles of cyclic voltammetry scans, much superior to the commercial Pt/C (20 wt.%). The combined experimental and theoretical studies reveal that the Pt&Ru serve as the main active sites for HER and the CoNiCu species modify the electron density of active sites to facilitate the H* adsorption and achieve an optimum M-H binding energy. The hierarchical pore structure and N-doping of hNCNC support also play a crucial role in the enhancement of HER activity and stability. This study demonstrates an effective strategy to greatly improve the HER performance of noble metals by developing the HEAs on the unique hNCNC support.

基于原位聚合凝胶电解质的碳纳米笼//三氧化钨纳米棒超级电容器

发展非对称超级电容器可有效提升超级电容器能量密度,选择电极材料和电解质是关键.分级结构碳纳米笼因具有比表面积大、微孔-介孔-大孔共存、导电性好、稳定性高等优点,特别适合用作超级电容器电极材料.进一步通过N,S共掺杂引入赝电容、改善浸润性,所得的氮硫共掺杂碳纳米笼(NSCNC)在1 mol•L^(−1)H2SO4溶液、电势范围0~1 V、电流密度1 A•g^(−1)下表现出337 F•g^(−1)的高比容量.水合三氧化钨(WO3•0.6H2O)纳米棒通过W6+/W5+的氧化还原反应实现H+的嵌入与脱出,在-0.55~0.3 V、5 A•g^(−1)下表现出454 F•g^(−1)的高比容量.以NSCNC和WO3•0.6H2O作正负极材料、原位聚合高分子凝胶电解质(IPGE/H2SO4)作准固态电解质组装的非对称超级电容器的工作电压为1.5 V,其倍率性能非常接近于在H型电解池中以1 mol•L^(−1)H2SO4为电解液的器件,而远优于以传统聚乙烯醇/硫酸(PVA/H2SO4)作凝胶电解质的器件,其根源是原位聚合的IPGE/H2SO4与电极材料之间建立了有效的电荷传输界面,改善了H+离子的传导,有效降低了电压降.本工作不仅展示了酸性介质中NSCNC//WO3•0.6H2O超级电容器的优异储能性能,还提供了一种新的用于构建准固态超级电容器的原位聚合凝胶电解质.

分级结构碳纳米笼高效催化苄胺氧化偶联制N-苄烯丁胺

苄胺氧化偶联制N-苄烯丁胺通常需使用贵金属催化剂,开发廉价催化剂具有重要研究价值.本工作以具有大比表面积和丰富表面缺陷的分级结构碳纳米笼(hCNCs)作为无金属催化剂,在无溶剂、100℃和常压O2条件下即可实现苄胺到N-苄烯丁胺的高效转化,反应8 h的苄胺转化率和N-苄烯丁胺选择性均可达98%,远优于碳纳米管、还原氧化石墨烯、活性炭等典型碳材料.hCNC700样品循环使用6次后催化性能基本无衰减,且具有优秀的底物拓展性.hCNC700的优异催化性能源于其超高的比表面积可提供大量的缺陷活性位点,而独特的分级孔结构十分有利于反应过程中的传质,使丰富的表面活性位点(缺陷)得以充分利用.

In situ construction of porous hierarchical(Ni3-xFex)FeN/Ni heterojunctions toward efficient electrocatalytic oxygen evolution

As a choke point in water electrolysis,the oxygen evolution reaction(OER)suffers from the severe electrode polarization and large overpotential.Herein,the porous hierarchical hetero-(Nis Fe)FeN/Ni catalysts are in situ constructed for the eficient electrocatalytic OER.X-ray absorption fine structure characterizations reveal the strong Ni-Fe bimetallic interaction in(Niz Fex)FeN/Ni.Theoretical study indicates the heterojunction and bimetallic interaction decrease the free-energy change for the rate-limiting step of the OER and the overpotential thereof.In addition,the high conductivity and porous hierarchical morphology favor the electron transfer,electrolyte access and O2 release.Consequently,the optimized catalyst achieves a low overpotential of 223 mV at 10 mA.cm^-2,a small Tafel slope of 68 mV:dec^-1,and a high stability.The excellent performance of the optimized catalyst is also demonstrated by the overall water electrolysis with a low working voltage and high Faradaic efficiency.Moreover,the correlation between the structure and performance is well established by the experimental characterizations and theoretical calculations,which confirms the origin of the OER activity from the surface metal oxyhydroxide in situ generated upon applying the current.This study suggests a promising approach to the advanced OER electrocatalysts for practical applications by constructing the porous hierarchical metal-compound/metal heterojunctions.

Advanced Ni-Nx-C single-site catalysts for CO2 electroreduction to CO based on hierarchical carbon nanocages and S-doping

Metal-nitrogen-carbon materials are promising catalysts for CO2 electroreduction to CO. Herein, by taking the unique hierarchical carbon nanocages as the support, an advanced nickel-nitrogen-carbon single-site catalyst is conveniently prepared by pyrolyzing the mixture of NiCl2 and phenanthroline, which exhibits a Faradaic efficiency plateau of > 87% in a wide potential window of −0.6 – −1.0 V. Further S-doping by adding KSCN into the precursor much enhances the CO specific current density by 68%, up to 37.5 A·g−1 at −0.8 V, along with an improved CO Faradaic efficiency plateau of > 90%. Such an enhancement can be ascribed to the facilitated CO pathway and suppressed hydrogen evolution from thermodynamic viewpoint as well as the increased electroactive surface area and improved charge transfer fromkinetic viewpoint due to the S-doping. This study demonstrates a simple and effective approach to advanced electrocatalysts by synergetic modification of the porous carbon-based support and electronic structure of the active sites.

Boosting oxygen reduction activity of spinel CoFe2O4 by strong interaction with hierarchical nitrogen-doped carbon nanocages

The unique hierarchical nitrogen-doped carbon nanocages(h NCNC) are used as a new support to homogeneously immobilize spinel Co Fe_2O_4 nanoparticles by a facile solvothermal method. The so-constructed hierarchical Co Fe_2O_4/h NCNC catalyst exhibits a high oxygen reduction activity with an onset potential of0.966 V and half-wave potential of 0.819 V versus reversible hydrogen electrode, far superior to the corresponding 0.846 and 0.742 V for its counterpart of Co Fe_2O_4/h CNC with undoped hierarchical carbon nanocages(h CNC) as the support, which locates at the top level for spinel-based catalysts to date.Consequently, the Co Fe_2O_4/h NCNC displays the superior performance to the Co Fe_2O_4/h CNC, when used as the cathode catalysts in the home-made Al-air batteries. X-ray photoelectron spectroscopy characterizations reveal the more charge transfer from Co Fe_2O_4 to h NCNC than to h CNC, indicating the stronger interaction between Co Fe_2O_4 and h NCNC due to the nitrogen participation. The enhanced interaction and hierarchical morphology favor the high dispersion and modification of electronic states for the active species as well as the mass transport during the oxygen reduction process, which plays a significant role in boosting the electrocatalytic performances. In addition, we noticed the high sensitivity of O 1 s spectrum to the particle size and chemical environment for spinel oxides, which is used as an indicator to understand the evolution of ORR activities for all the Co Fe_2O_4-related contrast catalysts. Accordingly,the well-defined structure-performance relationship is demonstrated by the combination of experimental characterizations with theoretical calculations. This study provides a promising strategy to develop efficient, inexpensive and durable oxygen reduction electrocatalysts by tuning the interaction between spinel metal oxides and the carbon-based supports.

Advanced non-precious electrocatalyst of the mixed valence CoO_x nanocrystals supported on N-doped carbon nanocages for oxygen reduction

Taking advantage of the nitrogen(N)-participation and large surface area of N-doped carbon nanocages(NCNCs),the Co Ox nanocrystals are conveniently immobilized onto the NCNCs with high dispersion.The Co Ox/NCNCs hybrid exists in the mixed valence with predominant Co O over Co3O4 and demonstrates superb oxygen reduction reaction activity and stability remaining^94%current density even after operation over 100 h.These results suggest a promising strategy to develop advanced electrocatalysts with the novel NCNCs or even beyond.

Mesostructured carbon-based nanocages: an advanced platform for energy chemistry

The electrochemistry in energy conversion and storage(ECS) not only relies on the active species in catalysts or energy-storage materials, but also involves mass/ion transport around the active species and electron transfer to the external circuit. To realize high-rate ECS process, new architectures for catalysts or energy-storage electrodes are required to ensure more efficient mass/charge transport. 3 D porous mesostructured materials constructed by nanoscale functional units can form a continuous conductive network for electron transfer and an interconnected multiscale pores for mass/ion transport while maintaining the high surface area, showing great promise in boosting the ECS process. In this review, we summarize the recent progress on the design,construction and applications of 3 D mesostructured carbon-based nanocages for ECS. The role of the hierarchical architectures to the high rate performance is discussed to highlight the merits of the mesostructured materials. The perspective on future opportunities and challenges is also outlined for deepening and extending the related studies and applications.

Iron oxide encapsulated in nitrogen-rich carbon enabling high-performance lithium-ion capacitor

Lithium-ion capacitors(LICs)could combine the virtues of high power capability of conventional supercapacitors and high energy density of lithium-ion batteries.However,the lack of high-performance electrode materials and the kinetic imbalance between the positive and negative electrodes are the major challenge.In this study,Fe3O4 nanoparticles encapsulated in nitrogen-rich carbon(Fe3O4@NC)were prepared through a self-assembly of the colloidal Fe OOH with polyaniline(PANI)followed by pyrolysis.Due to the well-designed nanostructure,conductive nitrogen-rich carbon shells,abundant micropores and high specific surface area,Fe3O4@NC-700 delivers a high capacity,high rate capability and long cycling stability.Kinetic analyses of the redox reactions reveal the pseudocapacitive mechanism and the feasibility as negative material in LIC devices.A novel LIC was constructed with Fe3O4@NC-700 as the negative electrode and expanded graphene(EGN)as the positive electrode.The wellmatched two electrodes effectively alleviate the kinetic imbalance between the positive and negative electrodes.As a result,Fe3O4@NC-700//EGN LIC exhibits a wide operating voltage window,and thus achieves an ultrahigh energy density of 137.5 W h kg^-1.These results provide fundamental insights into the design of pseudocapacitive electrode and show future research directions towards the next generation energy storage devices.

Boosting faradaic efficiency of CO_(2)electroreduction to CO for Fe-NC single-site catalysts by stabilizing Fe^(3+)sites via F-doping

The atomically dispersed Fe^(3+)sites of Fe-N-C single-site catalysts(SSCs)are demonstrated as the active sites for CO_(2)electroreduction(CO_(2)RR)to CO but suffer from the reduction to Fe^(2+)at~−0.5 V,accompanied by the drop of CO faradaic efficiency(FECO)and deterioration of partial current(JCO).Herein,we report the construction of F-doped Fe-N-C SSCs and the electron-withdrawing character of fluorine could stabilize Fe3+sites,which promotes the FECO from the volcano-like highest value(88.2%@−0.40 V)to the high plateau(>88.5%@−0.40-−0.60 V),with a much-increased JCO(from 3.24 to 11.23 mA·cm^(−2)).The enhancement is ascribed to the thermodynamically facilitated CO_(2)RR and suppressed competing hydrogen evolution reaction,as well as the kinetically increased electroactive surface area and improved charge transfer,due to the stabilized Fe^(3+)sites and enriched defects by fluorine doping.This finding provides an efficient strategy to enhance the CO_(2)RR performance of Fe-N-C SSCs by stabilizing Fe^(3+).

Ultrahigh rate capability of 1D/2D polyaniline/titanium carbide(MXene)nanohybrid for advanced asymmetric supercapacitors

High energy density and enhanced rate capability are highly sought-after for supercapacitors in today's mobile world.In this work,polyaniline/titanium carbide(MXene)(PANI/Ti3C2Tx)nanohybrid is synthesized through a facile and cost-effective self-assembly of.one-dimensional(10)PANI nanofibers and two-dimensional(20)Ti3C2Tx nanosheets.PANl!Ti3C2Tx delivers greatly improved specific capacitance,ultrahigh rate capability(67%capacitance retention from 1 to 100 A·g^(-1))as well as good cycle stability.Electrochemical kinetic analysis reveals that PANI/Ti3C2Tx is featured with surface capacitance-dominated process and has a quasi-reversible kinetics at high scan rates,giving rise to an ultrahigh rate capability.By using PANl!Ti3C2Tx as positive electrode,an 1.8 V aqueous asymmetric supercapacitor(ASC)is successfully assembled,showing a maximum energy density of 50.8 Wh·kg^(-1)·(at 0.9 kW-kg-1)and a power density of 18 kW·kg^(-1)(at 26 Wh·kg^(-1)).Moreover,an 3.0 V organic ASC is also elaborately fabricated,·by using PANI/Ti3C2Tx,achieving an ultrahigh energy density of 67.2 Wh·kg^(-1)(at 1.5 kW·kg^(-1))and a power density of 30 kW·kg^(-1)·(at 26.8 Wh·kg^(-1)).The present work not only improves fundamental understanding of the structure-property relationship towards ultrahigh rate capability electrode materials,but also provides valuable guideline for the rational design of high-performance:energy storage devices with both high energy and power densities.

Construction of hierarchical FeNi_(3)@(Fe,Ni)S_(2) core-shell heterojunctions for advanced oxygen evolution

The investigation of earth-abundant electrocatalysts for efficient water electrolysis is of central importance in renewable energy system, which is currently impeded by the large overpotential of oxygen evolution reaction (OER). NiFe sulfides show promising OER activity but are troubled by their low intrinsic conductivities. Herein, we demonstrate the construction of the porous core-shell heterojunctions of FeNi3@(Fe,Ni)S_(2) with tunable shell thickness via the reduction of hierarchical NiFe(OH)x nanosheets followed by a partial sulfidization. The conductive FeNi3 core provides the highway for electron transport, and the (Fe,Ni)S_(2) shell offers the exposed surface for in situ generation of S-doped NiFe-oxyhydroxides with high intrinsic OER activity, which is supported by the combined experimental and theoretical studies. In addition, the porous hierarchical morphology favors the electrolyte access and O_(2) liberation. Consequently, the optimized catalyst achieves an excellent OER performance with a low overpotential of 288 mV at 100 mA·cm^(−2), a small Tafel slope of 48 mV·dec^(−1), and a high OER durability for at least 1,200 h at 200 mA·cm^(−2). This study provides an effective way to explore the advanced earth-abundant OER electrocatalysts by constructing the heterojunctions between metal and corresponding metal-compounds via the convenient post treatment, such as nitridation and sulfidization.

Effective enhancement of electrochemical energy storage of cobalt-based nanocrystals by hybridization with nitrogen-doped carbon nanocages

Cobalt-based oxygenic compounds Co(OH)2,CoO and Co3 O4 are attractive for electrochemical energy storage owing to their high theoretical capacities and pseudocapacitive properties.Despite the great efforts to their compositional and morphological regulations,the performances to date are still quite limited owing to the low active surface area and sluggish charge transfer kinetics.Herein,different Co-based nanocrystals(Co-NCs)were conveniently anchored on the hierarchical nitrogen-doped carbon nanocages(hNCNCs)with high specific surface area and coexisting micro-meso-macropores to decrease the size and facilitate the charge transfer.Accordingly,a high specific capacity of1170 Fg^-1 is achieved at 2 Ag^-1 for the Co(OH)2/hNCNCs hybrid,in which the capacitance of Co(OH)2(2214 F gco(OH)2)is approaching to its theoretical maximum(2595 Fg^-1),demonstrating the high utilization of active materials by the hybridization with N-doped nanocarbons.This study also reveals that these Co-NCs store/release electrical energy via the same reversible redox reaction despite their different pristine compositions.This insight on the energy storage of Co-based nanomaterials suggests that the commonly-employed transformation of the Co-NCs from Co(OH)2 to CoO and Co3 O4 on carbon supports is unnecessary and even could be harmful to the energy storage performance.The result is instructive to develop high-energy-density electrodes from transition metal compounds.

Recent Advances in Non-Precious Metal-Nitrogen-Carbon Single-Site Catalysts for CO_(2)Electroreduction Reaction to CO

The carbon dioxide electroreduction reaction(CO_(2)RR)to fuels and/or chemicals is an efficient prospective strategy to realize global carbon management using intermittent electric energy harvested from renewable sources.Highly efficient inexpensive electrocatalysts are required to achieve high energy and Faradaic efficiencies as well as fast conversion.Metal-nitrogen-carbon(M-N-C)single-site catalysts(SSCs)are highly competitive over precious metal catalysts in the CO_(2)RR to CO due to their high performance,easy regulation and low cost.In the past six years,intensive studies of M-N-C SSCs for CO_(2)RR to CO have been performed,and great progress has been achieved.This review focuses on the important topic of CO_(2)RR to CO with M-N-C SSCs.We first introduce the reaction mechanism of the CO_(2)RR to CO and the regulation of the electronic structure from a theoretical viewpoint.Then,the construction of M-N-C SSCs and the regulation of the electronic structure are demonstrated experimentally.The up-to-date electrocatalytic performance of M-N-C SSCs with different metal centers(Ni,Fe,Co and others)is summarized and compared systematically to highlight structure-performance correlations that were considered from both theoretical and experimental perspectives.Finally,the opportunities,challenges and future outlooks are summarized to deepen and widen research and applications in this promising field.

Defect-induced deposition of manganese oxides on hierarchical carbon nanocages for high-performance lithium-oxygen batteries

The cathode of lithium-oxygen(Li-O_(2))batteries should have large space for high Li_(2)O_(2) uptake and superior electrocatalytic activity to oxygen evolution/reduction for long lifespan.Herein,a high-performance MnO_(x)/hCNC cathode was constructed by the defect-induced deposition of manganese oxide(MnOx)nanoparticles on hierarchical carbon nanocages(hCNC).The corresponding Li-O2 battery(MnOx/hCNC@Li-O_(2))exhibited excellent electrocatalytic activity with the low overpotential of 0.73-0.99 V in the current density range of 0.1-1.0 A·g^(-1).The full discharge capacity and cycling life of MnO_(x)/hCNC@Li-O_(2) were increased by~86.7%and~91%,respectively,compared with the hCNC@Li-O_(2) counterpart.The superior performance of MnO_(x)/hCNC cathode was ascribed to(i)the highly dispersed MnO_(x) nanoparticles for boosting the reversibility of oxygen evolution/reduction reactions,(ii)the interconnecting pore structure for increasing Li_(2)O_(2) accommodation and facilitating charge/mass transfer,and(iii)the concealed surface defects of hCNC for suppressing side reactions.This study demonstrated an effective strategy to improve the performance of Li-O_(2) batteries by constructing cathodes with highly dispersed catalytic sites and hierarchical porous structure.