Carbon nanotubes (CNTs) are of great interest for many potential applications because of their extraordinary electronic, mechanical and structural properties. However, issues of chaotic staking, high cost and high energy dissipation in the synthesis of CNTs remain to be resolved. Here we develop a facile, general and high-yield strategy for the oriented formation of CNTs from metal-organic frameworks (MOFs) through a low-temperature (as low as 430 °C) pyrolysis process. The selected MOF crystals act as a single precursor for both nanocatalysts and carbon sources. The key to the formation of CNTs is obtaining small nanocatalysts with high activity during the pyrolysis process. This method is successfully extended to obtain various oriented CNT-assembled architectures by modulating the corresponding MOFs, which further homogeneously incorporate heteroatoms into the CNTs. Specifically, nitrogen-doped CNT-assembled hollow structures exhibit excellent performances in both energy conversion and storage. On the basis of experimental analyses and density functional theory simulations, these superior performances are attributed to synergistic effects between ideal components and multilevel structures. Additionally, the appropriate graphitic N doping and the confined metal nanoparticles in CNTs both increase the densities of states near the Fermi level and reduce the work function, hence efficiently enhancing its oxygen reduction activity. The viable synthetic strategy and proposed mechanism will stimulate the rapid development of CNTs in frontier fields.
Rechargeable aqueous zinc-ion batteries are highly desirable for grid-scale applications due to their low cost and high safety; however, the poor cycling stability hinders their widespread application. Herein, a highly durable zinc-ion battery system with a NaVO·1.63HO nanowire cathode and an aqueous Zn(CFSO) electrolyte has been developed. The NaVO·1.63HO nanowires deliver a high specific capacity of 352 mAh g at 50 mA g and exhibit a capacity retention of 90% over 6000 cycles at 5000 mA g, which represents the best cycling performance compared with all previous reports. In contrast, the NaVO nanowires maintain only 17% of the initial capacity after 4000 cycles at 5000 mA g. A single-nanowire-based zinc-ion battery is assembled, which reveals the intrinsic Zn storage mechanism at nanoscale. The remarkable electrochemical performance especially the long-term cycling stability makes NaVO·1.63HO a promising cathode for a low-cost and safe aqueous zinc-ion battery.
Carbon materials are generally preferred as anodes in supercapacitors; however, their low capacitance limits the attained energy density of supercapacitor devices with aqueous electrolytes. Here, we report a low-crystalline iron oxide hydroxide nanoparticle anode with comprehensive electrochemical performance at a wide potential window. The iron oxide hydroxide nanoparticles present capacitances of 1,066 and 716 F g−1 at mass loadings of 1.6 and 9.1 mg cm−2, respectively, a rate capability with 74.6% of capacitance retention at 30 A g−1, and cycling stability retaining 91% of capacitance after 10,000 cycles. The performance is attributed to a dominant capacitive charge-storage mechanism. An aqueous hybrid supercapacitor based on the iron oxide hydroxide anode shows stability during float voltage test for 450 h and an energy density of 104 Wh kg−1 at a power density of 1.27 kW kg−1. A packaged device delivers gravimetric and volumetric energy densities of 33.14 Wh kg−1 and 17.24 Wh l−1, respectively.
Aqueous zinc-ion batteries attract increasing attention due to their low cost, high safety, and potential application in stationary energy storage. However, the simultaneous realization of high cycling stability and high energy density remains a major challenge. To tackle the above-mentioned challenge, we develop a novel Zn/VO rechargeable aqueous hybrid-ion battery system by using porous VO as the cathode and metallic zinc as the anode. The VO cathode delivers a high discharge capacity of 238 mAh g at 50 mA g. 80% of the initial discharge capacity can be retained after 2000 cycles at a high current density of 2000 mA g. Meanwhile, the application of a "water-in-salt" electrolyte results in the increase of discharge platform from 0.6 to 1.0 V. This work provides an effective strategy to simultaneously enhance the energy density and cycling stability of aqueous zinc ion-based batteries.
Developing efficient, stable, and low-cost catalysts for oxygen evolution reaction (OER) is highly desired in water splitting and metal−air batteries. Transition metal−organic frameworks (MOFs) have emerged as promising catalysts and have been intensively investigated especially due to their tunable crystalline structure. Unlike traditional strategies of tuning the morphology of well-crystalline MOFs, low-crystalline bimetallic MOFs are constructed via inducing exotic metal ions, and the formation process is revealed by experimental and theoretical methods. The lowcrystalline bimetallic MOFs exhibit rich active sites due to local crystallinity and long-range disorder and deliver a small overpotential of 260 mV at 10 mA cm −2 , a low Tafel slope of 35 mV dec −1 , and a high Faradaic efficiency of 99.5% as oxygen evolution elecctrocatalysts. The work opens up a new avenue for the development of highly efficient earth-abundant catalysts in frontier potential applications.
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