Efficient K-Storage of Fe-Coupled Organic Molecule Anode in Ether-Based Electrolytes.

Given these advantages of widely designable structures and environmentally friendly characteristics, organic electrode materials (OEMs) are considered to be promising electrode materials for alkaline metal-ion batteries. However, their large-scale application is hampered by insufficient specific capacity and rate performance. Here, Fe2+ is coupled to the anhydride molecule NTCDA to form a novel K-storage anode Fe-NTCDA. In this way, the working potential of Fe-NTCDA anode is reduced, which makes it more suitable to be used as an anode material. Meanwhile, the electrochemical performance is significantly improved due to the increase in K-storage sites. Moreover, electrolytes regulation is implemented to optimize the K-storage behavior, resulting into a high specific capacity of 167 mAh/g after 100 cycles at 50 mA/g and 114 mAh/g even at 500 mA/g in the 3 M KFSI/DME electrolytes.

Atomic-Scale Dispersed Fe-Based Catalysts Confined on Nitrogen-Doped Graphene for Li-S Batteries: Polysulfides with Enhanced Conversion Efficiency.

Lithium-sulfur batteries have been considered as potential electrochemical energy-storage devices owing to their satisfactory theoretical energy density. Nonetheless, the inferior conversion efficiency of polysulfides in essence leads to fast capacity decay during the discharge/charge cycle. In this work, it is successfully demonstrated that the conversion efficiency of lithium polysulfides is remarkably enhanced by employing a well-distributed atomic-scale Fe-based catalyst immobilized on nitrogen-doped graphene (Fe@NG) as a coating of separator in lithium-sulfur batteries. The quantitative electrocatalytic efficiency of the conversion of lithium polysulfides is determined through cyclic voltammetry. It is also proven that the Fe-NX configuration with highly catalytic activity is quite beneficial for the conversion of lithium polysulfides. In addition, the adsorption and permeation experiments distinctly indicate that the strong anchoring effect, originated from the charge redistribution of N doping into the graphene matrix, inhibits the movement of lithium polysulfides. Thanks to these advantages, if the as-prepared Fe@NG catalyst is combined with polypropylene and applied as a separator (Fe@NG/PP) in Li-S batteries, a high initial capacity (1616 mA h g-1 at 0.1 C), excellent capacity retention (93 % at 0.2 C, 70 % at 2 C), and superb rate performance (820 mA h g-1 at 2 C) are achieved.

Optimal Configuration of N-Doped Carbon Defects in 2D Turbostratic Carbon Nanomesh for Advanced Oxygen Reduction Electrocatalysis.

The charge redistribution strategy driven by heteroatom doping or defect engineering has been developed as an efficient method to endow inert carbon with significant oxygen reduction reaction (ORR) activity. The synergetic effect between the two approaches is thus expected to be more effective for manipulating the charge distribution of carbon materials for exceptional ORR performance. Herein we report a novel molecular design strategy to achieve a 2D porous turbostratic carbon nanomesh with abundant N-doped carbon defects (NDC). The molecular level integration of aromatic rings as the carbon source and urea units as the N source and sacrificial template into the novel precursor of polyurea (PU) promises the formation of abundant carbon edge defects and N doping sites. A special active site-a carbon edge defect doped with a graphitic valley N atom-was revealed to be responsible for the exceptional ORR performance of NDC material.

Controllable Construction of Core-Shell Polymer@Zeolitic Imidazolate Frameworks Fiber Derived Heteroatom-Doped Carbon Nanofiber Network for Efficient Oxygen Electrocatalysis.

Designing rational nanostructures of metal-organic frameworks based carbon materials to promote the bifunctional catalytic activity of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is highly desired but still remains a great challenge. Herein, an in situ growth method to achieve 1D structure-controllable zeolitic imidazolate frameworks (ZIFs)/polyacrylonitrile (PAN) core/shell fiber (PAN@ZIFs) is developed. Subsequent pyrolysis of this precursor can obtain a heteroatom-doped carbon nanofiber network as an efficient bifunctional oxygen electrocatalyst. The electrocatalytic performance of derived carbon nanofiber is dominated by the structures of PAN@ZIFs fiber, which is facilely regulated by efficiently controlling the nucleation and growth process of ZIFs on the surface of polymer fiber as well as optimizing the components of ZIFs. Benefiting from the core-shell structures with appropriate dopants and porosity, as-prepared catalysts show brilliant bifunctional ORR/OER catalytic activity and durability. Finally, the rechargeable Zn-air battery assembled from the optimized catalyst (CNF@Zn/CoNC) displays a peak power density of 140.1 mW cm-2 , energy density of 878.9 Wh kgZn-1 , and excellent cyclic stability over 150 h, giving a promising performance in realistic application.

MOF-Based Metal-Doping-Induced Synthesis of Hierarchical Porous CuN/C Oxygen Reduction Electrocatalysts for Zn-Air Batteries.

A transition-metal-nitrogen/carbon (TM-N/C, TM = Fe, Co, Ni, etc.) system is a popular, nonprecious-metal oxygen reduction reaction (ORR) electrocatalyst for fuel cell and metal-air battery applications. However, there remains a lack of comprehensive understanding about the ORR electrocatalytic mechanism on these catalysts, especially the roles of different forms of metal species on electrocatalytic performance. Here, a novel CuN/C ORR electrocatalyst with a hybrid Cu coordination site is successfully fabricated with a simple but efficient metal-organic-framework-based, metal-doping-induced synthesis strategy. By directly pyrolyzing Cu-doped zeolitic-imidazolate-framework-8 polyhedrons, the obtained CuN/C catalyst can achieve a high specific surface area of 1182 m2 g-1 with a refined hierarchical porous structure and a high surface N content of 11.05 at%. Moreover, regulating the Cu loading can efficiently tune the states of Cu(II) and Cu0 , resulting in the successful construction of a highly active hybrid coordination site of NCu(II)Cu0 in derived CuN/C catalysts. As a result, the optimized 25% CuN/C catalyst possesses a high ORR activity and stability in 0.1 m KOH solution, as well as excellent performance and stability in a Zn-air battery.

Screening Nickel-Doped Mo2 C Nanorod Arrays for Ultrastable and Efficient Hydrogen Evolution over a Wide pH Range.

Green hydrogen, using sustainable energy to decompose water to produce hydrogen, is regarded as the ideal and effective connection to convert electricity into chemical energy. Herein, well designed Ni-doped Mo2 C nanorod electrodes self-supported on three types of substrates (Ni foam, Cu foam and stainless steel wire mesh) with outstanding gas resistance and prominent corrosion resistance were assembled together to build up a wide pH applicable electrode for Hydrogen Evolution. In particular, Ni-doped Mo2 C nanorod arrays on stainless steel wire mesh donated as Ni-Mo2 C@SSW exhibited remarkable electrocatalytic properties towards hydrogen evolution reaction with superior overpotentials both in 1 M KOH and 0.5 M H2 SO4 (102 mV and 106 mV at the current density of 10 mA cm-2 ) and incomparable continuous durability. This work provides the possibility for the realization of low cost, high activity and ultra-stable durability HER electrocatalysts in practical industrial application.

Efficient yolk-shelled Fe-N-C oxygen reduction electrocatalyst via N-rich molecular-guided pyrolysis.

Fe-N-C catalysts with highly dispersed metal active centers were developed as promising non-precious metal materials for acidic oxygen reduction reaction (ORR) electrocatalysis. However, such kind of novel catalysts still suffer from major challenges in the manipulation of dispersion, utilization, and stability of the Fe-based metal centers. Herein, a N-rich molecular dual-guided pyrolysis strategy was proposed to develop an efficient yolk-shelled Fe-N-C ORR electrocatalyst. A unique yolk-shelled nanostructure with a relatively ordered shell and disordered yolk of a carbon skeleton was controllably constructed via this guided-pyrolysis route from the precursor of Fe-doped zeolitic imidazolate framework-8 (Fe-ZIF-8). Moreover, the atomic-level dispersion of Fe element in the carbon skeleton could be achieved via the dual guidance from phenanthroline and melamine molecules. The optimized Fe-N-C catalyst demonstrated a half-wave potential of 0.78 V vs. RHE in acid media, close to commercial 30% Pt/C, along with a small negative shift of 19 mV after an accelerated durability test. These enhanced electrocatalytic properties could be attributed to the preferred transformation of the Fe precursors to atomically dispersed Fe-Nx active configurations, as well as the enhanced three-phased interfacial reaction kinetics.

Flame-Retardant Nonaqueous Electrolytes for High-Safety Potassium-Ion Batteries.

Potassium-ion batteries (PIBs) with conventional organic-based flammable electrolytes suffer from serious safety issues with a high risk of ignition and burning especially under harsh conditions, which significantly limits their widespread applications. Flame-retardant electrolytes (FREs) are considered as one of the most effective strategies to address these safety issues. Therefore, it's much necessary to summarize the challenges, recent progress, and design principles of flame-retardant nonaqueous electrolytes for PIBs to guide their development and future applications. In this review, an in-depth introduction and explanation of the origins of electrolyte flammability are first presented. Particularly, the state-of-the-art design principles of FREs for PIBs are extensively summarized and emphasized, including the electrolyte flame-retardant solvents/additives, highly concentrated electrolytes (HCEs), localized high-concentration electrolytes (LHCEs), ionic liquids-based electrolytes and solid-state electrolytes. Moreover, the advantages and drawbacks of each approach are systematically presented and discussed, following by proposed perspectives to guide the rational development of next-generation high-safety PIBs for practical applications.

Defective Carbon Derived Using a Dissolution-Recrystallization Strategy for Oxygen Reduction Electrocatalysis.

Dopant-free defective carbon electrocatalysts have been considered as promising alternatives to traditional precious metal electrocatalysts recently. Compared with precious metal catalysts and transition-metal catalysts, since there are no metals doped, electrochemical devices assembled with dopant-free defective carbons are free from environmental pollution and subsequent recovery problems. In order to obtain abundant carbon defects with high-intrinsic catalytic activity, the synthesis of dopant-free defective carbons requires complex and harsh preparation conditions. Therefore, the construction of active defects with efficient utilization, especially through a simple process, is still a great challenge for the development of dopant-free defective carbon electrocatalysts. Herein, dissolution-recrystallization strategy was employed to design Zn-MOF-74 precursors for the synthesis of dopant-free defective carbons, realizing the synchronous manipulation of high ratio of carbon defects and highly exposed mass transfer channels. One-dimensional porous defective carbon nanorods (d-CNRs), which exhibited excellent oxygen reduction reaction (ORR), electrocatalytic activity, and molecular selectivity, were synthesized by directly carbonizing rodlike Zn-MOF-74 precursors. Attributed to the dissolution-recrystallization strategy, with the activation of in situ-formed ZnO, the synthesized d-CNRs exhibited unique pore-crack nested porous structures, which carried abundant defects as activity sites for ORR and showed a surprisingly high specific surface area of 2459 m2/g with a high ratio of mesopores. d-CNRs also showed promising applications in Zn-air batteries with a stable long-term discharge of no obvious voltage drop after 60 h. The dissolution-recrystallization strategy provided a simple controllable pathway for the efficient construction of dopant-free defective carbon electrocatalysts.

Manipulating K-Storage Mechanism of Soft Carbon via Molecular Design-Driven Structure Transformation.

The emerging potassium-ion batteries (PIBs) have been placing stratospheric expectations for realizing grid-scale electrochemical storage of renewable energy. However, the unsatisfactory K-storage of PIB anode materials, especially promising carbonaceous materials, significantly limited the development of PIBs. Here, a molecular design strategy was proposed to realize controllable structure transformation of soft carbon (SC) materials for enhanced K-storage performance. The optimized SC-PCN material delivered a high reversible K-storage capacity of 838 mAh/g at 50 mA/g, outstanding rate capability (213 mAh/g at 1000 mA/g), and excellent long-term cycling performance (301 mAh/g maintained after 300 cycles at 500 mA/g), superior to most previously reported carbon-based PIB anodes materials. Reaction kinetic analysis revealed that the proposed molecular design strategy can achieve the transformation from a surface capacitive-dominated mechanism to a capacitive-diffusion hybrid mechanism for SC-PCN, benefiting from its unique microstructures with highly defective surface generated via the synergistic effect from template removal, N doping, and surface reconstruction. The optimal hybrid K-storage mechanism should be responsible for the excellent K-storage properties of the prepared SC-PCN.

Electrolyte Regulation for Non-Graphitic Carbon to Achieve Stable Long-Cycling K-Storage.

Potassium-ion batteries have been considered as a promising next-generation energy storage system due to low cost but comparable energy density to lithium-ion batteries. However, carbon-based anode materials usually delivered unsatisfactory K-storage capacity as well as long-cycling performance due to poor matching with common electrolytes, thus forming an unstable solid electrolyte interphase (SEI). Herein, a robust KF-rich SEI can be achieved on the as-prepared non-graphitic carbon surface by regulating the electrolyte solvation structures, which can significantly suppress redox reaction of solvents and ensure highly reversible K+ intercalation/deintercalation. As a result, the as-synthesized non-graphitic carbon anode predictably exhibits super long-cycling performance with about 200 mA h/g at 100 mA/g for 1000 cycles and a stable capacity of 135 mA h/g at 500 mA/g for 2000 cycles with negligible capacity decay in the optimized 3 M KFSI/DME electrolyte. This work provides deep insights into further development and improvement of advanced electrolyte systems for next generation energy storage devices.

Decoupling layer metal-organic frameworks via ligand regulation to achieve ultra-thin carbon nanosheets for oxygen reduction electrocatalysis.

2D imidazole MOFs are considered to be ideal carbon precursors for oxygen reduction reactions owing to their adjustable ligand components and durable coordination mode. Due to the massive electron delocalization in the lamella, the conjugative effect among 2D MOF layers immensely restricts the exposure of catalytic sites after carbonization, which makes the decoupling layer extremely important on the premise of ensuring activity. Herein, atomic thickness ultra-thin zinc-imidazole MOF precursors were prepared through a bottom-up ligand regulated strategy to achieve the aim of lamellar decoupling. The introduction of heterologous ligands excites stable delocalized electrons, resulting in a decrease in the interlayer force of 2D zinc-imidazole MOF precursors. Subsequent salt template-supported ammonia pyrolysis assisted the MOF-derived carbon sheets to grow along the transverse direction and optimize pore size distribution as did the doping nitrogen type. The MOF-derived carbon sheets demonstrated increasing mesopores and fringe graphitic N which could significantly promote the mass transfer and electron transfer speed during the oxygen reduction reaction. In addition, the obtained ultra-thin carbon delivered an outstanding onset potential (0.98 V vs. RHE) and durability (retaining 91% of the initial current after 12000 s of operation), showing tremendous commercial prospects in sustainable energy.

Balance of N-Doping Engineering and Carbon Chemistry to Expose Edge Graphitic N Sites for Enhanced Oxygen Reduction Electrocatalysis.

Nitrogen-doped nanocarbon materials (NCMs) have been developed as promising metal-free oxygen reduction reaction (ORR) electrocatalysts. However, insufficient attention on the balance of N-doping engineering and carbon chemistry significantly suppressed the revelation of the real active configurations as well as the ORR mechanism for NCMs. Herein, 1,4-phenylenediurea (BDU) with multifunctional blocks was designed for the synthesis of NCMs, realizing synchronous manipulation of N-doping engineering and carbon chemistry. The good balance between N-doping engineering (especially graphitic edge N configurations) and carbon chemistry (including the specific surface area, porosity distribution, and graphitization degree) at a pyrolysis temperature of 1000 °C resulted in the best ORR performance for obtaining N-doped carbon nanorod (NCR) materials. A general descriptor χ was then proposed for evaluating the balance states between N-doping engineering and carbon chemistry. The prediction of the ORR performance of NCMs from their physical properties as well as searching for the optimal active configuration from the relationships between ORR performance and different configurations can be realized from such a practical descriptor, which can also be extended to other nanocarbon-based metal-free electrocatalytic reactions for deeply understanding their electrocatalytic mechanisms.

Transformation of ZIF-8 nanoparticles into 3D nitrogen-doped hierarchically porous carbon for Li-S batteries.

Li-S batteries have been attracting increasing interest owing to their remarkable advantages of low cost, high theoretical capacity and high theoretical energy density. Nevertheless, the severe "shuttle effects" of lithium polysulfides have markedly limited the performance of the cells and further hindered their commercial applications. Herein, a novel scheme combining a transformation strategy with ammonia treatment was developed to fabricate ZIF-8-derived nitrogen-doped hierarchically porous carbon (NHPC/NH3). When NHPC/NH3 was used as a host of sulfur, the obtained S@NHPC/NH3 cathode for Li-S cells presented an initial specific capacity of 1654 mA h g-1 and an outstanding cycling stability with only 0.27% attenuation per cycle from the 30th cycle to 130th cycle. Together with the theoretical calculation, it was concluded that such excellent electrochemical performances should be attributed to the suppressed "shuttle effect" via both physical and chemical adsorption of lithium polysulfides in the optimized microporous structures with effective nitrogen doping sites as well as the improved kinetics owing to the abundant meso/macroporous structures.

Stepwise Fabrication of Co-Embedded Porous Multichannel Carbon Nanofibers for High-Efficiency Oxygen Reduction.

A novel nonprecious metal material consisting of Co-embedded porous interconnected multichannel carbon nanofibers (Co/IMCCNFs) was rationally designed for oxygen reduction reaction (ORR) electrocatalysis. In the synthesis, ZnCo2O4 was employed to form interconnected mesoporous channels and provide highly active Co3O4/Co core-shell nanoparticle-based sites for the ORR. The IMC structure with a large synergistic effect of the N and Co active sites provided fast ORR electrocatalysis kinetics. The Co/IMCCNFs exhibited a high half-wave potential of 0.82 V (vs. reversible hydrogen electrode) and excellent stability with a current retention up to 88% after 12,000 cycles in a current-time test, which is only 55% for 30 wt% Pt/C.

Interconnected Hierarchically Porous Fe, N-Codoped Carbon Nanofibers as Efficient Oxygen Reduction Catalysts for Zn-Air Batteries.

Developing porous carbon-based non-precious-metal catalysts for an oxygen reduction reaction (ORR) is a suitable approach to significantly reduce the costs of fuel cells or metal-air batteries. Herein, interconnected hierarchically porous carbon nanofibers simultaneously doped with nitrogen and iron (HP-Fe-N/CNFs) were fabricated by facile pyrolysis of polypyrrole-coated electrospun polystyrene/FeCl3 fibers. The obtained carbon nanofibers have a high specific surface area (569.6 m2/g) and large pore volume (1.00 cm3/ g) as well as effective doping of N and Fe. Benefiting from the improved mass transfer and utilization of active sites attributed to interconnected hierarchical porous structures, HP-Fe-N/CNFs display excellent ORR catalytic activity in alkaline media, with a comparable onset potential and half-wave potential but superior selectivity, stability, and tolerance against methanol to commercial 30 wt % Pt/C. Particularly, when applied in an assembled Zn-air battery, HP-Fe-N/CNFs outperform 30 wt % Pt/C in power density and long-term stability, explicitly showing their promising practical application.

Bottom-up synthesis of high-performance nitrogen-enriched transition metal/graphene oxygen reduction electrocatalysts both in alkaline and acidic solution.

Oxygen reduction electrocatalysts with low cost and excellent performance are urgently required for large-scale application in fuel cells and metal-air batteries. Though nitrogen-enriched transition metal/graphene hybrids (N-TM/G, TM = Fe, Co, and Ni and related compounds) have been developed as novel substitutes for precious metal catalysts (PMCs) towards oxygen reduction reaction (ORR), a significant challenge still remains for simple and efficient synthesis of N-TM/G catalysts with satisfactory electrocatalytic behavior. Herein, we demonstrate a universal bottom-up strategy for efficient fabrication of strongly-coupled N-TM/G catalysts. This strategy is implemented via direct polymerization of transition metal phthalocyanine (TMPc) in the two-dimensional confined space of in situ generated g-C3N4 and a subsequent pyrolysis. Such a space-confined bottom-up synthesis route successfully constructs a strongly-coupled triple junction of transition metal-graphitic carbon-nitrogen-doped graphene (TM-GC-NG) with extensive controllability over the specific surface area, nitrogen content/types as well as the states of metal. As a result, the optimized N-Fe/G materials have promising potential as high-performance NPMCs towards ORR both in alkaline and acidic solution.

In Situ Self-Sacrificed Template Synthesis of Fe-N/G Catalysts for Enhanced Oxygen Reduction.

To facilely prepare high-performance Fe-N/G oxygen reduction catalysts via a simple and controllable route from available and low-cost materials is still a challenge. Herein, we develop an in situ self-sacrificed template strategy to synthesize Fe-N/G catalysts from melamine, glucose, and FeSO4·7H2O. Fe/Fe3C@graphitic carbon nanocapsules are uniformly formed on the NG surface to create a highly opened and stable mesoporous framework structure. Furthermore, effectively doped N sites and high active Fe-Nx sites are synchronously constructed on such structures, leading to an enhanced synergistic effect for ORR and promising the Fe-N/G catalyst a similar catalytic activity and four-electron selectivity, but superior stability to commercial 30 wt % Pt/C catalysts in 0.1 M KOH solution under the same loading.